Sunlight-based large area light source and large area luminaire

ABSTRACT

A light source ( 25 ) for emitting collimated light ( 29 ) in particular for a large area luminaire ( 21 ) comprises a light guide unit ( 43 ) for guiding light by total internal reflection. The light guide unit comprises a plurality of localized light source regions ( 57 ) at a main front face ( 55 A) for having light pass there through. The light source ( 25 ) further comprises a plurality of light emitting units ( 41 ) for emitting light into the light guide strips ( 91 ) through respective portions of the at least one coupling face ( 47 ) of the light guide unit ( 43 ), and a collimation unit ( 45 ) extending along the main front face ( 55 A) and comprising a plurality of collimating elements. At least one light emitting unit ( 41 ) is configured as a light input coupling assembly ( 250 ) to receive collected natural light from a fiber ( 249 ) and to provide the received natural light to the light guide unit ( 43 ). The light source ( 25 ) can be implemented in a sunlight-based illumination system ( 241 ) collecting and providing natural light to the light source ( 25 ).

TECHNICAL FIELD

The present disclosure relates generally to luminaires, and inparticular to a light source for luminaires intended, for example, forimitating natural sunlight illumination. Moreover, the presentdisclosure relates generally to generating a light beam from a largearea with high brightness and homogeneity of the luminance across thearea.

BACKGROUND

Artificial lighting systems for closed environments often aim atimproving the visual comfort experienced by users. In particular,lighting systems are known which imitate natural lighting, specificallysunlight illumination, in particular using light with a high correlatedcolor temperature (CCT), and a large color rendering index (CRI). Thecharacteristics of such outdoor lighting to be imitated depend on theinteraction between the sunlight and the earth atmosphere and create aspecific shade characteristic.

EP 2 304 478 A1, EP 2 304 480 A1, WO 2014/076656 A1, WO 2014/075721 A1as well as PCT/EP2014/000835 filed on 27 Mar. 2014, all filed by thesame applicants, disclose lighting systems with a light source producingvisible light in particular in form of a low-divergent light beam, and apanel containing nanoparticles. During operation of the lighting system,the panel receives the light from the light source and acts as aso-called Rayleigh diffuser, namely it diffuses light rays similarly tothe earth atmosphere in clear-sky conditions. Specifically, the conceptuses directed light, which corresponds to sunlight and generates shadowsin presence of lit objects, and diffused light with a larger CCT, whichcorresponds to the light of the blue sky.

For providing cool white light as well as warm white light, LED basedlight sources may be used that are based on, for example,phosphor-converted white LEDs and/or a combination of various colorLEDs. The optical properties of light emitted from LEDs require beamshaping optical configurations, usually collimating optics such as lensand/or mirror systems. An exemplary light source configured as a lightbeam projector is disclosed in PCT/EP2014/001293 filed on 13 May 2014 bythe same applicant.

Furthermore, luminaires intended for providing a skylight appearance aredisclosed in US 2014/0321113 A1 and US 2014/0160720 A1. In the latter,light is coupled out of a light guide through the complete surface areaof the light guide in essentially all directions. A covering plate canbe passed through holes provided in the plate such that localizedemission spots are provided across the plate. The blue content of lightpassing under larger angles is increased by a blue reflecting conceptincluding the inner side of the covering plate at the regions betweenthe holes, through which no light is emitted.

For completeness, it is further referred to the Applicants' ownapplication PCT/EP2015/069790, filed on 28 Aug. 2015.

Daylight-based illumination systems collect daylight and guide thecollected light in particular by optic systems to luminaires that aredistributed within a building. Accordingly, daylight-based illuminationsystems are energy efficient and allow the illumination of an indoorambience with light having a natural visible spectrum.

Commercially available products include lens-based or reflector-basedcollector optics, a tracking system for following the sun-movement withthe collector optics, a fiber-based light distribution system, andindoor light providing units. Daylight based illumination systems aredisclosed, for example, in US 2004/187908 A1 and WO 2008/143586 A1. Anexample of non-patent disclosure is the publication “Towards hybridlighting systems: A review”, by M. S. Mayhoub et al., Lighting Res.Technol. 2010, 42: 51-71.

The collector optics is, for example, a parabolic reflector-based systemusing a parabolic primary mirror arrangement to focus the sunlight intoa plurality of fibers, typically after a secondary reflection.Alternative embodiments use a plurality of lenses to collect thesunlight and to focus it into respective fibers. The tracking of theoptimal light collecting conditions is usually based on sensor systemstracking the position of the sun and/or on geo-location systems, therebyoptimizing the amount of collected light.

The light distribution system is usually based on fiber bundlesreceiving and guiding the collected light essentially without lossesover significant distances. Losses are primarily generated byinterfaces, each creating back scattering and Fresnel losses. Additionallosses are created inside the fiber by light absorption. Accordingly,the number of interfaces are kept at a minimum value and the typicalvalue of the length of the fibers is selected in the range of, forexample, 10 m to 15 m.

The present disclosure is directed, at least in part, to improving orovercoming one or more aspects of prior systems. In particular, lightsources and luminaires for replicating sunlight and skylight are ofinterest that have a form factor suitable for high volume and low costmanufacturing.

SUMMARY OF THE DISCLOSURE

In a first aspect, a light source for emitting collimated light, inparticular for an edge-lit large area luminaire, comprises a light guideunit having a main front face, a main back face, and at least onelateral coupling face connecting the main front face and the main backface in a thickness direction. The light guide unit is configured forguiding light received at the at least one lateral coupling face bytotal internal reflection between the main front face and the main backface. The main front face comprises a plurality of localized lightsource regions that are areas with a limited extent in two-dimensionsfor having light pass there through, wherein the light source regionsare surrounded by a non-source region. The light source comprisesfurther a plurality of light emitting units for emitting light into thelight guide unit through the at least one coupling face, wherein atleast one light emitting unit is configured as a light input couplingassembly to receive collected natural light from a fiber and to providethe received natural light to the light guide unit. The light sourcecomprises further collimation unit extending along the main front facethat comprising a plurality of collimating elements, wherein eachcollimating element comprises an input side and an output side, isoptically associated to one of the plurality of light source regions, isconfigured to receive light emerging from the associated light sourceregion at its input side, and to emit collimated light from a respectivecollimated light emitting region formed at its output side.

In another aspect, a sunlight-based illumination system is disclosed forproviding a direct light beam, in particular for generating a sun-likeappearance within a sun-sky imitating illumination. The sunlight-basedillumination system comprises a sunlight receiving unit with a collectorsystem, a plurality of optical fibers, wherein the collector system isconfigured for collecting natural outdoor light, and for coupling thecollected light into the plurality of optical fibers and each of theplurality of optical fibers comprises a fiber output end. Thesunlight-based illumination system comprises further a light source asdescribed above for emitting collimated light, in particular for a largearea luminaire, wherein the fiber output ends of the plurality ofoptical fibers are coupled to light input coupling assemblies such thatcollected natural light is coupled into the light guide unit andcontributes to the direct light beam.

The following aspect may similarly be combined with the herein disclosedconcepts of using collected natural light and/or liquid crystal basedchromatic diffusing layer. For example, the aspect of a light source foremitting collimated light is disclosed that, in particular, may be for alarge area luminaire such as an edge-lit large area luminaire. The lightsource comprises a light guide unit comprising a plurality of lightguide strips, wherein the light guide strips define together, for thelight guide unit, a main front face, a main back face, and at least onecoupling face. For example, a coupling face may connect the main frontface and the main back face in a thickness direction. For the lightsource, each light guide strip of the plurality of light guide strips isconfigured for guiding light received at the at least one coupling face.The light may be guided, for example, by total internal reflection. Eachlight guide strip comprises a plurality of localized light sourceregions at the main front face for having light pass there through,wherein the light source regions are provided along the light guidestrip within a non-source region. The light source comprises further aplurality of light emitting units for emitting light into the lightguide strips through respective portions of the at least one couplingface, and a collimation unit extending along the main front face andcomprising a plurality of collimating elements. Each collimating elementcomprises an input side and an output side, is optically associated toone of the plurality of light source regions, and is configured toreceive light emerging from the associated light source region at itsinput side and to emit collimated light from a respective collimatedlight emitting region formed at its output side.

In another aspect, a light source for emitting collimated light inparticular for a, for example edge-lit, large area luminaire comprises alight guide unit having a main front face, a main back face, and atleast one coupling face, which may be a lateral coupling face connectingthe main front face and the main back face in a thickness direction. Thelight guide unit is configured for guiding light received at the atleast one coupling face, for example by total internal reflectionbetween the main front face and the main back face. Its main front facecomprises a plurality of localized light source regions for having lightpass there through, wherein the light source regions are providedsubstantially uniform within a non-source region. The light sourcefurther comprises a plurality of light emitting units for emitting lightinto the light guide unit through the at least one coupling face, and acollimation unit extending along the main front face and comprising aplurality of collimating elements, wherein each collimating elementcomprises an input side and an output side, is optically associated toone of the plurality of light source regions, is configured to receivelight emerging from the associated light source region at its inputside, and to emit collimated light from a respective collimated lightemitting region formed at its output side.

In another aspect, a light source for emitting collimated light inparticular for a (e.g. edge-lit) large area luminaire comprises a lightguide unit comprising a plurality of light guide strips defining a mainfront face, a main back face, and at least one coupling face (e.g.configured as a lateral coupling face connecting the main front face andthe main back face in a thickness direction. Each light guide strip ofthe plurality of light guide strips is configured for guiding lightreceived at the at least one coupling face e.g. by total internalreflection. The light source comprises further a plurality of lightemitting units for emitting light into the light guide strips throughrespective portions of the at least one coupling face, and a collimationunit extending along the main front face and comprising a plurality ofcollimating elements, wherein each collimating element comprises acompound parabolic concentrator or TIR lens having an input sideoptically coupled to one of the plurality of light guide strips suchthat a plurality of light source regions are formed along the respectivelight guide strips at which the compound parabolic concentrators or TIRlenses receive light from the light guide strips, and each collimatingelement is further configured to emit collimated light from a respectivecollimated light emitting region formed at its output side.

In another aspect, a light source for emitting collimated light inparticular for a large area luminaire comprises a light guide unitcomprising a plurality of light guide strips defining a main front face,a main back face, and at least one coupling face such as a lateralcoupling face connecting the main front face and the main back face in athickness direction. Each light guide strip of the plurality of lightguide strips is configured for guiding light received at the at leastone lateral coupling face e.g. by total internal reflection, andcomprises a plurality of localized light source regions at the mainfront face for having light pass there through. The light source furthercomprising a plurality of light emitting units for emitting light intothe light guide strips through respective portions of the at least onecoupling face and the light emission extends in the thickness directionover an angular range of below 40° such as below 200, e.g. about 10°,around an input light central direction. The light source furthercomprises a collimation unit extending along the main front face andcomprising a plurality of collimating elements, wherein each collimatingelement comprises an input side and an output side, is opticallyassociated to one of the plurality of light source regions, and isconfigured to receive light emerging from the associated light sourceregion at its input side and to emit collimated light from a respectivecollimated light emitting region formed at its output side.

In some embodiments, the light emitting units may be configured to emitprimary light with a spectral distribution that compensates for spectrallosses accumulated by the primary light while propagating within thelight guide unit, and/or the light emitting units may comprise subgroupsfor generating counter-propagating light within the light guide unit tocompensate for losses accumulated by the primary light while propagatingwithin the light guide unit and to provide for comparable lightextraction at comparable light source regions such that the luminanceper area section of the light emitting face is essentially homogeneous.

In another aspect, a large area luminaire, for example, an edge-litlarge area luminaire comprises a light source as described above, and achromatic diffusing layer comprising a plurality of nanoparticlesembedded in a matrix and configured to provide for a direct transmissionthat is larger in the red than in the blue and for a diffusetransmission that is larger in the blue than in the red, wherein thechromatic diffusing layer is in particular positioned to by illuminatedby the collimated light or is positioned downstream of the light sourceregions such as downstream of the input side and/or the output side ofthe collimation unit.

In some embodiments of the luminaire, the chromatic diffusing layer maybe provided as a panel that has a back side provided at a light emittingface of the light source for being illuminated by incident light, andwherein in particular there may be an air gap between the collimatingunit, and/or wherein the collimation unit may comprise a firstcollimating element layer and a second collimating element layer and thechromatic diffusing layer may be positioned between the firstcollimating element layer and the second collimating element layer, orwherein the collimation unit may comprise a first collimating elementlayer and a second collimating element layer that is located downstreamof the first collimating element layer and comprises the matrix and theplurality of nanoparticles, or wherein the collimation unit may comprisea coating with the matrix and the plurality of nanoparticles applied toa light emitting face formed by surfaces associated with the collimatedlight emitting regions.

It is noted that the features recited for one or more independent claimsas dependent claims will equally apply to other aspects as disclosed inthe description or listed in the claims. Other features and aspects ofthis disclosure will be apparent from the following description and theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and constitutea part of the specification, illustrate exemplary embodiments of thedisclosure and, together with the description, serve to explain theprinciples of the disclosure. In the drawings:

FIG. 1 is a schematic illustration of the concept of condensed sunlightimitating luminaires;

FIG. 2A to FIG. 2C are graphs illustrating design considerations forlight sources of collimated light;

FIG. 3A and FIG. 3B are schematic front and cut views of a condensedsunlight imitating luminaire;

FIG. 3C is a schematic 3D-view of a room being illuminated with aluminaire such as the one of FIGS. 3A and 3B;

FIG. 3D is a schematic illustration of the appearance of the luminaireof FIGS. 3A and 3B when looked at from below;

FIG. 3E is a schematic illustration of an exemplary sunlight-basedillumination system;

FIGS. 3F and 3G are schematic illustrations of sunlight collection;

FIG. 3H is a schematic drawing illustrating the coupling of naturallight into a condensed sunlight imitating luminaire;

FIG. 4A to FIG. 4I and FIG. 4K to FIG. 4M are schematic illustrations ofexemplary coupling configurations of primary light sources and lightguide units;

FIG. 5A to FIG. 5C are schematic illustrations of light extraction fromthe light guide unit in a reflective mode;

FIG. 6A to FIG. 6D are schematic illustrations of light extraction fromthe light guide unit in a transmission mode;

FIG. 6E and FIG. 6F are schematic illustrations of baffleconfigurations;

FIG. 7A to FIG. 7C are illustrations of configurations for illustratingcolor tuning features;

FIG. 8A is a schematic illustration of a portion of a light guide unitcomprising light guide strips;

FIG. 8B to FIG. 8D are schematic illustrations of light sources beingbased on light guide strips;

FIG. 9A to FIG. 9C illustrate configurations using primary light that isin a fan-like manner, and in particular tilted with respect to a centralaxis of the light guide unit, provided the light guide unit; and

FIG. 10A to FIG. 10D illustrate an exemplary configuration withspecifically shaped reflective extractors within, for example, a lightguide strip.

DETAILED DESCRIPTION

The following is a detailed description of exemplary embodiments of thepresent disclosure. The exemplary embodiments described therein andillustrated in the drawings are intended to teach the principles of thepresent disclosure, enabling those of ordinary skill in the art toimplement and use the present disclosure in many different environmentsand for many different applications. Therefore, the exemplaryembodiments are not intended to be, and should not be considered as, alimiting description of the scope of patent protection. Rather, thescope of patent protection shall be defined by the appended claims.

The present disclosure is based in part on the aspects relating toconfiguration of a (thin) light guide (herein also referred to aswaveguide approach) with output providing light beam emission with avery small beam divergence (such as below 4°), possibly with acircularly symmetric emission (such as light emission within a cone).

For example, the disclosure is based in part on the realization that awaveguide approach might address the following aspects: Efficiency,i.e., how can light be coupled into as well as guided as well as coupledout from the light guide unit efficiently because absorption (of thematerial, of reflective surfaces, etc.) and losses (scattering,imperfections of the surfaces, spurious reflections) may significantlyreduce the light output. Collimation: i.e., how can light be extractedfrom the light guide unit while obtaining a very small beam divergence.It is noted that herein the term collimation or to collimate areunderstood as to refer to light that is of low divergence incorrespondence to the specific embodiment and, for example, luminaireimplementation. In some embodiments, a low divergent beam emitted from alight source may have a full-width beam angle in the range of (below) 5°such as 4° or below, e.g. 2°, while in some embodiments it may be in therange of (below) 10° such as 7° such as 4° or below, e.g. 2°.

Regarding, for example, the above aspects, the inventors realized interalia that—instead of letting the beam propagate inside the light guideunit within the angular range with respect to the guide direction of ±θ,with θ being at the most the complementary angle of the TIR angle (theTIR angle in PMMA, considering n=1.4936, is roughly 42°)—light may becoupled into the light guide unit in form of a narrower angular fan. Forexample, inside the guide medium the angular fan may spread over ±15°,such as ±10°, or even ±5°, or even less. The angular fan may be centeredat an angular direction such that the fan is close to the edge of thelight guide acceptance range. The inventors realized that theprobability of interaction of the light propagating inside the lightguide unit with the surface on which the light extracting features areorganized (e.g. in an array structure) is increased.

Moreover, the inventors realized that—instead of considering a fullplanar light guide unit, the light guide unit may comprise (be dividedinto) a series of parallel “strips”. Each strip receives primary lightfrom LED sources positioned at one or both ends at their extrema. Theextracting features may be a single line array along the strip. Also thestrip configuration increases the probability of interaction of thelight propagating inside the light guide unit with the surface on whichthe light extracting features are positioned.

In general, the inventors realized solutions that enhances theprobability of interaction with the extracting features of the lightguide unit, thus reducing the path of the light rays inside the volumeof the guide. This further reduces absorptions from the material andfrom the guide extrema (which are not 100% reflective).

Moreover, the inventors realized that chromatic shifts in the outputspectrum due to absorptions can be compensated by properly tailoring theinput spectrum of the light propagating within the guide.

Finally, the inventors realized that light guides or light guidingstrips can be coupled to fibers providing collected sunlight as anenergy efficient light source.

In the following, background considerations of etendue conservation inthe context of sun-sky-like illumination concepts are given that form abasis for the various aspects of light sources and luminaires disclosedherein.

Referring to FIG. 1, the sun 1 is 149 600 000 km from the earth and hasa diameter of 1 391 684 km, which results in an approximate full-widthbeam angle of about 0.53° for direct solar radiation at the earth'ssurface (exemplary indicated by a window 3). As described in the abovereferenced patent disclosure of the applicant, light from the sun at theearth can be replicated by combining a high-intensity light source 5with a properly designed Rayleigh-like acting diffuser 7. Assuming alight source with a standard 2′ (0.61 m)×4′ (1.22 m) aperture (area:0.74 m²), which is designed to approximate direct sunlight, the etendueof the light source is supposed to be ≤0.74 m² π sin(0.53/2)² sr˜50 mm²sr. If the light source were single and based on a chip-on-board (COB)style light-emitting diode (LED), which provides a Lambertian circularemitter, the allowable light-emitting area would be 50 mm² sr/π sr=˜16mm², meaning the light emitting surface (LES) diameter of the LED wouldbe 4.5 mm.

Standard light output from a 2′×4′ aperture is usually at least 3000 lm(see DesignLights Consortium™ Product Qualification Criteria, Table 4:Primary Use Requirements). In order to provide a reasonable level oflight output, and accounting for reasonable system optical losses, onemay need to select a larger COB LED than 4.5 mm LES. For example, NichiaCorporation's NFDWJ130B may provide more than 6000 lm with an LESdiameter of 14.6 mm, or etendue of 167 mm² it sr=526 mm² sr. Due toetendue constraints such a source could best achieve a beam half-angledivergence of 0.86° assuming the above aperture size, i.e. larger thanthat of direct solar radiation but still very narrow and potentiallyadequate for imitating sun illumination.

Collimating the Lambertian light output of the COB LED for use in alighting system may be based on known collimating optical solutions suchas lenses or compound parabolic concentrator (CPC) or total internalreflection (TIR) optics, herein also referred to as CPC lens and TIRlens, respectively. For a full CPC, the optical element dimensions aredetermined by the relation L=cot(β_in/2) (a+b)/2, where L is the opticalelement length, a and b are the diameters of the input and outputsurfaces, and β_in/2 is the maximum allowed divergence half-angle insidethe CPC. The maximum half-angle β_out/2 exiting the CPC is then given byn sin (β_in/2)=sin(β_out/2). From etendue conservation, an area of theCPC's output surface is about 1/sin(0.86)²=˜4400 times the area of theinput surface, or a diameter of about 1 m. Ignoring the fact that theoptical element is a bit too large for the 2′×4′ aperture, the length ofthe CPC lens would be more than practically feasible.

The inventors noticed that the total area from which light is emittedfrom the LED (only about 170 mm²) is much smaller than that of theaperture (0.74 m²). As shown in FIG. 1, an optical system may be formedcomprising an array of multiple light emitting units 11 that togetheraccomplish simulating the sun. As an intermediate step illustrating thecondensation of the lay out, a configuration with a small number ofsources (here four sources 9) is shown in FIG. 1. Similar concepts weredisclosed in the above referenced WO 2014/075721 A1, which isincorporated herein by reference.

For the embodiment with a large number of light emitting units 11,because any one light emitting source is much smaller than the totalarea of the light emitting face of the optical system, the thicknesswill be reduced with respect to the case of a single CPC lens.

Following the etendue-based design considerations described above, FIG.2A illustrates the dependence of the length of a CPC lens vs. theindividual light source diameter. The dependence is exemplarily givenfor target beam divergences of 0.5, 1°, 2°, and 4°. The dependency isconsidered in the designs of a direct sunlight luminaire disclosedherein. For example, FIG. 2A indicates that the diameter of 50 μm forlight source regions may allow a 2° beam divergence with a CPC length ofabout 120 mm. Those dimensions indicate already the potential of adesign of a flat luminaire system with an overall thickness less thansix inches (152 mm), which is very attractive from a cost(bill-of-materials, BOM) and installation point of view.

As shown in FIG. 2B, the total length of the collimation unit may beshortened significantly by replacing a full CPC (consisting only of aCPC that provides for the respective collimation) with a combination ofcollimating optical elements comprising a CPC with large divergenceangle (such as 30° or even 45° or more) and a collimating lens such as aFresnel lens (herein referred to as CPC-lens collimator). In this case,the limited concentration effect of the CPC is acceptable as part of thecollimation is performed by the collimation lens For example, therequired length for the collimating unit as discussed above inconnection with FIG. 2A is reduced from 120 mm to about 8 mm, therebyenabling even lower-profile sun/sky luminaires that would be veryattractive in the marketplace.

In FIG. 2C, the dependence of diameter of a full CPC lens vs. sourcesize and beam divergences is illustrated. Similar dimensions will applyto a CPC-lens collimator discussed above in light of etendueconservation considerations. This information provides for the expectedarea of the collimated light emitting regions downstream of thecollimation unit. The inventors realized that the use of thisinformation is important because stray light or no light from regionsoutside the main narrow beams (i.e. non-collimated light or dark areasaround the collimated light emitting regions) may adversely affect theimpression of depth and the simulation of sunlight. Such stray light mayoccur at the edges of optical elements. Thus, the inventors realizedthat the pitch or scale of stray light is tied to the collimatingoptical element lens diameter. Specifically, stray light could beperceivable with luminance periodicities linked to the pitch ofcollimation optics (but not limited to this).

Thus, in addition to the aspects of FIGS. 2A and 2B, the teaching ofFIG. 2C may be used to design a sunlight luminaire according to theherein disclosed principles. In particular, one may design a system inwhich stray light occurs over a sufficiently small pitch. For example,for typical viewing distances such as 1 m to 5 m, the inventors havenoticed that stray light at pitches of a few millimeters or less doesnot appreciably detract from the eyes' view of the image of the “sun” asoccurring at an infinite distance, which is an important feature whensimulating the sun. FIG. 2C shows that for a 2° beam divergence, a CPCexit diameter of 2.9 mm can be achieved with an optical source size ofabout 50 μm diameter. Similarly, a CPC exit diameter of 5.8 mm may beachieved for an optical source size of about 100 μm diameter. It isnoted that in some surroundings smaller (elevator: several tencentimeter to about a meter) or larger (large halls: up to 10 m andmore) viewing distances may occur, where small viewing distances aremore challenging from the optical design perspective.

From the above optical source sizes on the order of, for example, 50 μmto 100 μm in lateral extension may form a basis to provide a luminairethat simulates direct sunlight and that is suitably thin and can beproduced by potentially low cost. However, LEDs commonly have LESdimensions larger than 200 μm. Even a 200×200 μm² LED chip solution,targeting a beam divergence of 2°, may result in a luminaire thicknesslarger than 0.5 m for a full CPC collimation unit and would have straylight spatial variation over distances of about 1 cm (corresponding tothe size of the CPC exit diameter and, thus, essentially setting thepitch between optical sources), which might reduce the sun simulationeffect. Thus, smaller light sources or smaller light source regions aredesirable.

The inventors have realized that it is possible to transform opticalsources such as LEDs from one form factor into another by utilizingdown-conversion and/or scattering features and/or reflecting features,herein also referred to as extractors. Exemplary extractors are disks ofluminescent down-converting phosphor compounds. For example, grains ofluminescent down-converting phosphor compounds may be mixed into asuitable binder (e.g., silicone) and dispensed (e.g., by a dispensermachine) onto a surface in the form of a disk. Light from a primarylight emitting unit (e.g. an LED or laser diode, LD) can bedirected/fall onto the disk. The disk of down-converting phosphorcompounds itself may then become a new optical source. In general such anew optical source is referred to herein as a localized light sourceregion that is energized by the light of the primary source (herein alsoreferred to as light emitting unit). That localized light source regionmay have the desired size for collimation in a thin optical structure asdiscussed above.

Suitable down-converting materials include phosphors, phosphorcompounds, organic light emitting materials, semiconductor materials,and semiconductor nanoparticles (such as “quantum dots”). A list ofcommon down-conversion materials is disclosed in US 20150049460 A, someare exemplarily listed in Table I below.

In an exemplary embodiment, the disk of down-converting phosphorcompounds may be deposited on either side of a light guide plate (LGP).LGPs are commonly used in the flat panel display industry, where theymay include primary light sources properly coupled to its edge(s).However, those flat panel displays may include a high density ofbuilt-in light-extraction or light-scattering features for lightemission (for example, the size of those features is comparable (and notmuch smaller as in the present disclosure) to the distance betweennearest neighbors). In contrast, LGPs for the herein disclosed conceptsare configured to guide light from primary light sources to specificextraction positions of the localized light source region (e.g. disks).

Primary light sources include herein sources of artificially generatedlight as well as sources of collected natural light such as may be usedfor sunlight-based illumination systems.

In the above exemplary embodiment and provided that the edges of the LGPare made reflective (e.g. using a reflective film), light in the LGP mayremain substantially trapped until it encounters a disk. Then, secondary(down-converted) light (if any) and scattered primary light from thedisk are emitted in a more or less Lambertian radiation pattern, meaningthat a significant fraction of that light will escape the LGP eitherdirectly at the disk or on the opposite side. That light would beassociated with a source size determined by the size of the disk.Assuming that many such disks may be provided, light from the primarylight sources coupled into the LGP at the edge(s) may be transformedinto light originating from an array of very small disks.

Extending the above exemplary embodiment, the small disks may be coupled(e.g. one to one) to an array of collimating elements, e.g. a CPC or aCPC-lens collimator as described above or a TIR lens or TIR lens-lenscollimator, thereby forming a compact luminaire that simulates sunlight.In particular, the output area (diameter for a circular shape) of thecollimating element sets the pitch of the extractor features/disksapplied to the LGP to ensure a uniform and homogenous emission of lightover the light emitting face of the light source. The input area of acollimating element is configured to receive a substantial amount oflight from the respective light source region (for example more than50%, such as more than 80%, or even more than 90%).

In the above exemplary embodiment, the down-converting phosphorcompounds may be selected and adjusted to provide for a sun-like lightspectrum. However, the inventors realized further that down-conversionmaterials may be omitted if the respective sun-like spectrum can begenerated by combining multiple light sources of varying colors. Forexample, the primary light sources might comprise blue-, green-, red-,or even white-emitting LEDs. In this case, the light extractors may bescattering features such as rough surface regions or regions of changesin refractive index that spoil the transmission of the LGP in a definedand localized manner. The inventors have realized that this may providemeans for uniform light mixing of light sources of different colors, andin extremely small form factors, and, combined with the teaching above,may lead to the realization of very narrow-divergence light beams withcolor tuning options. For example, the relative light outputs of theprimary light sources may be varied either by amplitude or pulse-widthmodulation in color and/or intensity. In some embodiments, the effect ofscattering centers and the effect of down-converting materials may evenbe combined. The latter might be useful in cases in which the downconversion materials are not inherently scattering, such as in the caseof semiconductor nanoparticles often called “quantum dots”.

Moreover, the inventors realized further that down-conversion materialsmay be omitted if collected natural light is used or the variousapproaches may be used in combination.

By assuming etendue conservation, the ratio between the area of inputface and the output face, e.g. as described below the ratio between the(combined) areas of the light source regions and the respective areacovered by the light guide (e.g. the area of the main front face of alight guide panel or the aperture size for a light guide stripconfiguration) are the (inverse of the) ratio between the solid anglesof the light in correspondence of the input and output faces (r=(0.5*sinα_out)²/n²(0.5*sin α_in)².

In the following, the ratio is indicated for the input divergence of theprimary light source (outside the light guide), the respective inputdivergence within the light guide, and the target output divergence of2° and 4°, respectively. A refractive index of 1.493 for the light guideand full angular divergences were considered. As will be understood bythe skilled person, the angular divergence within the light guide unitneeds to be applied for the etendue conservation considerations.Moreover, it is assumed that the angular content is uniformlydistributed at the input face; because the light source regions are atthe front surface of the light guide. It is noted that in someembodiments, the light source regions may not necessarily correspond tothe region of extractive interaction (see reflective structure 117 ofe.g. FIG. 5B and description of respective embodiments) such that theangular content may have developed to no longer be evenly distributed.The latter is one example why, even if etendue is conserved, the area ofthe respective source regions is larger than required by etendueconservation considerations. The ratio given in the overview below referto that small area instead of the potentially larger area of extractiveinteraction.

Input divergence Output divergence Input divergence (inside medium)(outside medium) Ratio (outside medium) α_in α_out r Lambertian  ~84° 2°0.03% Lambertian  ~84° 4° 0.12% 60°  39° 2° 0.11% 60°  39° 4° 0.45% 40°26.5° 2° 0.25% 40° 26.5° 4° 1.01% 20° 13.4° 2°   1% 20° 13.4° 4°   4%15°  10° 2° 1.78% 15°  10° 4° 7.12% 10°  6.7° 2°   4% 10°  6.7° 4°  16% 7°  4.7° 2°  8.2%  5° 3.35° 2°  16%

As can be seen, the ratio can be increased by using less divergent lightbeing coupled into the light guide. The ratio, thus, depends on theinput divergence and the targeted output divergence and lies in therange of below 0.1% up to 10% or even up to 16% for the indicatedparameters. It is noted that below-Lambertian input divergences apply inparticular for white light emitting primary light sources using CPCcoupling such as for down-converted LEDs, while the combination of laserdiodes with down-conversion materials at the entrance side of thecollimating element will essentially correspond to Lambertian emitters.For LED based embodiments, for example, a ratio of up to 10% may allowconfigurations providing the desired low-divergent light beam.

It is noted that in embodiments that allow providing some kind ofpre-collimated light at the light source regions, the etendueconsiderations summarized above may include that reduced etendue as astarting point. Accordingly, the skilled person will acknowledge thatconfigurations with a smaller pitch may be possible that still providefor an acceptable beam half-angle divergence. This may apply, forexample, to the embodiments using a reduced fan LED (resulting in lowangular ranges in the light guide), laser diodes, and/or focusingextractors.

In the following, at first an overview of a general configuration of aluminaire based on the herein disclosed concepts is described. Thenvarious embodiments of portions such as the light source, the underlyinglight coupling into and extraction from the light guide unit areillustrated.

FIGS. 3A and 3B illustrate a luminaire 21 as it may be mounted at, forexample, a wall 23 (ceiling or side wall) of a room in a building.Luminaire 21 comprises a light source 25 and a chromatic diffusing layer27 that together provide for a sun-like illumination. Specifically,light source 25 provides for an array of direct light beams 29, a partof which is transformed into a blue sky-like diffuse light 31 bychromatic diffusing layer 27, the remaining part passes through thechromatic diffusing layer 27 being perceived as a sun ray fallingthrough a window in wall 23.

In general, luminaire 21 may comprise a housing 33 covering and holdingtogether the various components as an exemplary mount. As shown in FIG.3B, in some embodiments, housing 33 may delimit an aperture 35 of theluminaire 21 through which the array of direct light beams 29 and bluesky-like diffused light 31 can be enter the room. In some embodiments,the luminaire 21 may be mounted within wall 23 and/or may have thehousing covering only the lateral sides and the back side.

A power supply and/or a control unit 34 may be provided within thehousing or—as shown in FIG. 3B—may be positioned externally to housing33.

In FIGS. 3A and 3B, components of an exemplary inner structure of lightsource 25 comprise a plurality of light emitting units 41, a light guideunit 43, and a collimation unit 45.

Specifically, light emitting units 41 act as primary light sources andare configured for emitting light into light guide unit 43 through atleast one lateral coupling face 47 of light guide unit 43 in a lightemitting direction 49. A light emitting unit 41 may be an artificiallight generating unit or a collected natural light providing units.Light emitting direction 49 is understood herein as the direction from arespective one of the plurality of light emitting units 41 to lightguide unit 43 as exemplarily indicated for two light emitting units inFIG. 3A.

In FIG. 3A, light emitting units 41 are grouped in two subgroups 51A and51B. For each subgroup 51A and 51B, the respective light emitting units41 are distributed along a respective lateral side of light guide unit43 and emit light along the direction in which the longitudinal sides oflight guide unit 43 extend. Subgroups 51A and 51B may be mounted torespective boards 53A, 53B that are connected to power supply and/orcontrol unit 34 and may comprise electronic devices configured tocontrol the output parameters of light emitting units 41 such asintensity and color spectrum, depending on the type of light emittingunits 41.

In general, light guide unit 43 has a main front face 55A, a main backface 55B that are connected by lateral coupling faces 47 in general in athickness direction dr. Light guide unit 43 is configured for guidinglight received at the at least one lateral coupling face by totalinternal reflection (TIR) between main front face 55A and main back face55B. Light guide unit 43 may be, for example, a high index waveguide.

As will be understood, within light guide unit 43, light propagates fromopposing lateral sides towards the respective other lateral side atwhich some of the light may be reflected back. In general, the lightdistribution within light guide unit 43 depends on the positions oflight emitting units 41, the type of coupling into light guide unit 43,and the light propagation conditions within light guide unit 43 such astotal internal reflection, absorption effects and reflectivity of thelateral sides, as well as the light extraction provided to extract lightfrom light guide unit 43.

In the embodiment of FIG. 3A, a panel-like shape of light guide unit 43is indicated but also other shapes such as triangular orstrip-configurations may be possible as will be described below. Ingeneral, a thickness of light guide unit 43 may be in the range from 1mm to 5 mm or even less in particular in strip-configurations.

In FIG. 3A, a grid of 5 times 3 localized light source regions 57 isindicated exemplarily. However, in general large numbers of up toseveral 10 thousand light source regions 57 may be present (e.g. an“emitter grid” of 88 000 of 100 μm extracting features on a 2′×4′ panelfor 4° final divergence and about 22 000 extracting features for a 2°final divergence). At localized light source regions 57, light isextracted from light guide unit 43 to enter light collimation unit 45.Light source regions 57 are evenly distributed over light guide unit 43at least within aperture 35. For example, light source regions 57 aredistributed over a non-source region 59 at equal distances in lightemitting direction 49. In light source 25, non-source region 59 is anarea of the main front face 55A of light guide unit 43 that does notessentially contribute light for illumination. For example, the lightguide unit 43 is configured such that essentially no light leaks out atthat non-source region 59. If any light does leave light guide unit 43from the non-source region 59 e.g. by leakage caused by some scatteringetc. that light is either so low in intensity or distributed over alarge angular range that it is not contributing to the perception orthat it may be blocked by baffle structures as disclosed herein. Forexample, the total flux (power) from the non-source region 59 may beabout or less than 10%, about or less than 5%, or even about or lessthan 2% of the flux originating from all light source regions, asdefined below. Moreover, the local flux density (in average) of thenon-source region 59 may about or less than 3% such as about or lessthan 1% or even several magnitudes less than the flux density (inaverage) emitted from the light source regions. The distance betweenneighboring light source regions may, for example, be in the range from0.5 mm to 15 mm such as about 3 mm or about 6 mm (herein also referredto as the pitch distance usually present in a light propagationdirection, wherein the light propagation direction coincides inparticular with the direction of a central axis of the light guide unit,and in particular the length dimension of a light guide strip). In lightof the small size of each light source region, the ratio of the area ofthe plurality of light source regions 57 with respect to the area of themain front face 55A (or: the area of the non-source region 59) may beless or equal to, for example, 0.2%. It is noted that, with respect tothe strip configurations disclosed herein, the non-source region maycomprise sections associated with the light guide strip as well as areasbetween light guide strips.

Some type of change in light propagation conditions will result inhaving light pass through main front face 55A at light source regions57. The passing light may be a portion of the primary light, i.e. thelight of light emitting units 41 (artificial light generating units orcollected natural light providing units), or the light may bedown-converted light e.g. generated by interaction of the primary lightwith a phosphor compound or a quantum-wall structure. Exemplaryembodiments will be described below. In other words, light sourceregions 57 may be considered to be defined by the light passing througha region—either due to the fact that light is incident on awall/interface forming the light guide unit with non-TIR-conditions dueto scattering, reflection, down-conversion etc., or by changing/removingTIR-conditions at a localized area of the wall/interface forming thelight guide unit.

As indicated above, the ratio of the area of the plurality of lightsource regions 57 with respect to the area of main front face 55A (orsimilarly the area of non-source region 59) depends on the angulardistribution of the light being provided to a collimating element (e.g.as emitted from the light source region) and the intended collimationangle. The ratio may differ thus significantly for fixed collimationangles of e.g. 2° or 4° and primary light conditions such as the angularcontent (fan) of the light guided within the light guide unit. ForLambertian input light to the collimating elements, the ratio of thearea of the plurality of light source regions 57 with respect to thearea of main front face 55A may be, for example, 0.5% or 0.2% or less.However, for cases in which the light source regions emit narrow-solidangle light, the area of the of the area of the plurality of lightsource regions 57 and, thus the ratio, can increase substantially, forexample, up to 10% or even up to 15% or 16% for large scale embodiments.Exemplary light source region may have diameters of about 3 mm arrangedat a pitch of 10 mm (corresponding e.g. to the diameter of thecollimating element at the exit side). In general, the smaller the ratiobetween the angles of the angular distribution being provided to thecollimating element and the intended collimation angle, the smaller isthe ratio between the areas of the light source regions and the mainfront face. Those considerations assume that the etendue is conserved inthe optical designs.

Typical values for the above ratio may be below 0.5% such as thementioned 0.2%, or even below 0.1%. As a further example, for a 100 μmlight source region and a 2.9 mm collimator (pitch), the ratio is thesame as for a single collimating element: 7850 μm²/8.41 mm² (i.e. about0.1%), assuming that the complete light source does not have any “lost”areas between collimating elements (such as areas 199 in FIG. 8B).

In general, the size of a light source region 57 may have a lateralextension in the range from 10 μm to 500 μm such as 100 μm. Moreover, alight source region 57 may have a circular, elliptic, rectangular, orsquare shape or in principle any shape; however, the shape may influencethe appearance of the light source in the far field. On contrastthereto, the thickness extension of light guide unit 43 may be is in therange from 1 mm to 5 mm, in some cases less than 1 mm. Light sourceregions 57 may be provided with the same size or essentially similar insize. Providing such light source regions 57 for the plurality ofcollimating elements may ensure similar input conditions for thereceived light at the input sides of the collimating elements.

In general, emission through the non-source region of the light guideunit is avoided or at least reduced to not spoil the perception of thesky-sun-imitation.

Accordingly, collimation unit 45 is configured to receive the light fromlight source regions 57 and, in general, to provide collimated light,e.g. having a divergence of few degrees to imitate sunlight.

Collimation unit 45 extends along main front face 55A and has a shapeadapted to the shape of light guide unit 43 and in particular to thedistribution of light source regions 57 and the shape of aperture 35.

Collimation unit 45 comprises a plurality of collimating elementscorresponding to the grid of light source regions 57. The thickness ofcollimation unit 45 and thus collimating elements may be in the rangefrom 1 mm to 0.3 m such as 2 mm to 15 cm.

Each collimating element is optically coupled to one of the plurality oflight source regions 57. Thus, each collimating element is configured toreceive (essentially only) light emerging from a respective light sourceregion 57 and to emit collimated light from a respective collimatedlight emitting region 61. Collimated light emitting regions 61 may havea lateral extension similar to the pitch distance of the light sourceregions 57, e.g. in the range from 0.5 mm to 50 mm, and for wellcollimated primary light such as light from diode lasers even below suchas 0.2 mm. Collimated light emitting regions 61 form a light emittingface 63 of light source 25. Face 63 may be a continuous surface, forexample, in embodiments in which collimating units are formed of a lensarray structure or a refractive CPC array structure. In embodiments inwhich collimating units are formed of, for example, a reflective CPCarray structure, face 63 may not be a surface as although lightcollimating regions 61 are not associated with a surface structure butinstead may be associated with the respective output sides of the CPCarray structure.

To perform the collimation, the collimating elements may comprise one ormore optical elements such as a lens and/or a CPC lens. The collimatingelements may form together, for example, a lens layer/CPC structure oftransparent materials such as PMMA (which could also be used for thelight guide unit and/or the matrix of the chromatic diffusing layer).

Light emitting face 63 may comprise a respectively small portion ofnon-emitting regions 65, e.g. at the boundaries between light emittingregions 61 (in FIG. 3A indicated as grid lines separating light emittingregions 61 such as transition or junction between lenses or refractiveCPCs or the walls of reflective CPCs).

The emission of each light emitting region 61 is directed light having abroad spectrum (e.g. white light) and a defined small divergence arounda main light beam propagation axis 67 as schematically indicated in FIG.3B for that part of the directed light having passed through chromaticdiffusing layer 27. Specifically, the emitted light distributes over anemission solid angle, thereby forming a light beam propagating along thedirection of main light beam propagation axis 67. FIG. 3B illustratesexemplarily divergent (slowly diverging) direct light beams 29 in thefar field. The far field depends on the near field as generated by therespective collimating element and the light being processed by thecollimating element.

In the far field, the local propagation direction across divergent lightbeams 29, i.e. a propagation direction of the directed non-diffusedlight, varies in dependence of the position within the cross-section ofdirect light beam 29 with respect to propagation axis 67. Specifically,a localized propagation direction 69 is increasingly inclined withrespect to main light beam propagation axis 67 with increasing distancefrom the inner area. Exemplarily, a maximum angle α_out/2 is indicatedin FIG. 3B for localized propagation direction being the furthest out,which corresponds to a beam divergence (herein also referred to as beamfull-angle divergence or total angular spread in the far field) of forexample, 2° or 4° of direct light beam 29. For a light beam 29 of asingle light source region (e.g. switching on just one light sourceregion) the far field may already be essentially formed at a distanceequal to the distance ceiling-floor (which may be an indoorluminaire-target) or even at smaller distances for more localizedillumination. This is shown in FIG. 3C by indicating light beam 29 asessentially circular. With respect to the main light beam of an entireluminaire (being composed of many light source regions und thusextending over a much larger area), the far field may be considered tohave formed at a much larger distance, e.g. of many tens of meters. Inthe far field, the essentially rectangular spot indicated by line 72 inFIG. 3C becomes circular as it loses the memory of the shape of theaperture. It is noted that the distance to form the far field of themain light beam depends on light angular divergence and luminaire size.

For a homogeneous emission, the area ratio between non-emitting region65 and collimated light emitting regions 61 is essentially constant overlight emitting face 63 of light source 25. Such a constant ratio maycontribute to a homogenous light emission from light emitting face 63.Moreover, the emission from one collimated light emitting region 61 maybe configured to be perceived as completely contained laterally (fullyflashed) inside the collimated light emitting region 61. Assuming asymmetric beam divergence of direct light beams 29, the juxtaposition ofcollimated light emitting regions 61 is then perceived by an observer,looking at them from within the light beam, as a bright circular diskimage, the diameter of which is related to the total angular spread inthe far field, divided into a set of “pixels” corresponding to thecollimated light emitting regions 61. The dimension of the singlecollimated light emitting regions 61 should be such that possible lightmodulation within the region is unlikely to be perceived by an observerlooking at them from a typical viewing distance.

As further schematically indicated in FIG. 3B, collimation unit 45 maycomprise some type of baffle structure 71 to decrease or even to avoidthe interaction of light between, in particular neighboring, collimatingelements downstream collimated light emitting regions 61 (herein alsoreferred to as cross-talk between individual light beam generatingoptical sequences—including the light source region, and collimatingelement). In some embodiments, baffle structure 71 may be, for example,a structural element (e.g. a shielding plate or surface of an opticalelement) that is coated with a light absorbing and/or darklayer/material or that has a dark finish.

With respect to the chromatic diffusing layer of the reflectivestructural unit, the present disclosure relates to an optical diffuseras disclosed in WO 2009/156348 A1 or international patent applicationentitled “TUNABILITY IN SUN-LIGHT IMITATING LIGHTING SYSTEMS”, filed on19 Nov. 2016, filed by the same applicants, which are incorporatedherein by reference.

In some embodiments, it may comprise an essentially transparent solidmatrix in which a plurality of solid transparent nanoparticles aredispersed, e.g. in a thin film, coating, or bulk material such assandwich embodiments. In the present description the terms “diffusinglayer”, “nanodiffuser”, and “chromatic diffusing layer” designate ingeneral an optical element, which comprises a matrix embedding those(essentially transparent) nanoparticles.

The chromatic diffusing layer is in principle capable of (chromatically)separating different chromatic components of incident light having abroad spectral bandwidth (such as in general white light) according tothe same mechanism that gives rise to chromatic separation in nature.Rayleigh scattering is creating, for example, the spectral distributioncharacteristic of skylight and sunlight. More particularly, thechromatic diffusing layer is capable of reproducing—when subject tovisible white light—the simultaneous presence of two different chromaticcomponents: a diffused sky-like light, in which blue—in other words theblue or “cold” spectral portion—is dominant, and a transmitted light,with a reduced blue component—in other words the yellow or “warm”spectral portion.

Referring to the transmission properties of a chromatic diffusing layerof luminaire 21, its structure is such that it achieves—based on thenanoparticles—such a specific optical property that comprises atransmission that is larger in the red than in the blue, and a diffusescattering that is larger in the blue than in the red. The opticalproperty may be present over essentially the complete range of aperture35.

Herein, as defined in the Standard Terminology of Appearance, ASTMinternational, E 284-09a, the transmittance is in general the ratio ofthe transmitted flux to the incident flux in the given conditions. Forexample, the diffuse transmittance is a property of the respectivespecimen that is given by the ratio of the transmitted flux to theincident flux, where the transmission is at all angles within thehemisphere bounded by the plane of measurement except in the directionof the regular transmission angle. Similarly, the regular transmittanceis the transmittance under the undiffused angle, i.e. the angle ofincidence. In the context of the present disclosure, for a givenwavelength and a given position on the chromatic diffusing layer, thediffuse transmittance and the regular transmittance are intended fornon-polarized incident light with an incident angle corresponding to themain light beam propagation axis 67. For measurements, the angular sizeof the detector for the measurement of transmission and the angularaperture of the incident beam is selectable in a range as it will beapparent to the skilled person. In particular when considering (whitelight) low angle diffusers, for example, the angular size of thedetector for the measurement of the regular transmittance and theangular aperture of the incident beam should be configured so that thesensor accepts rays with a transmission within a cone around theincident angle. In some embodiments, an angular aperture of 2 times 0.9°may be used.

Moreover, the transmitted flux is averaged over all possible incidenceazimuthal angles. In case the measurement of the diffuse transmittanceand/or the regular transmittance is hindered by geometrical or otherphysical constraints related to the configuration of the luminaire, theskilled person may have access to the above mentioned quantities byforming at least one separate chromatic diffusing layer and measuringthe transmittance directly for that section. For details of microscopicstructural properties, it is referred to, for example, the abovementioned publication WO 2009/156348 A1. However different values ofmicroscopic parameters may be applicable. For example, one may applyparameters that lead to a larger amount of scattered light with respectto non-scattered light. Similarly, in the aim of minimizing or at leastreducing the visibility of a possible stray light, one may preferincreasing the contribution to the luminance of the chromatic diffusinglayer due to diffused light in spite of the fact that the resultingperceived color may depart from the color of a perfect clear sky. Thelatter may be caused by reducing the level of color saturation as aconsequence of the multiple scattering arising therein and may be evencaused at concentrations below the concentration giving rise to multiplescattering.

In the following, some microscopic features are summarized exemplarily.

The chromatic effect is based on nanoparticles having a size in therange from, for example, 10 nm to 240 nm. For example, an average sizemay be in that range.

It is well known from fundaments of light-scattering that a transparentoptical element comprising transparent matrix and transparentnanoparticles having different refraction index with respect to thematrix, and having sizes (significantly) smaller than visiblewavelength, will preferentially scatter the blue part (the blue) of thespectrum, and transmit the red part (the red). While thewavelength-dependence of the scattering efficiency per single particleapproaches the λ⁻⁴ Rayleigh-limit law for particle sizes smaller orabout equal to 1/10 of the wavelength λ, a respective acceptable opticaleffect may be reached already in the above range for the size of thenanoparticles. In general, resonances and diffraction effects may startto occur at sizes larger, for example, half the wavelength.

On the other side, the scattering efficiency per single particledecreases with decreasing particle size d, proportional to d⁻⁶, makingthe usage of too small particle inconvenient and requiring a high numberof particles in the propagation direction, which in turn may be limitedby the allowed filling-fraction. For example, for thick scatteringlayers, the size of the nanoparticles embedded in the matrix (and inparticular their average size) may be in the range from 10 nm to 240 nm,such as 20 nm to 180 nm.

In some embodiments, larger particles may be provided within the matrixwith dimensions outside that range but those particles may not affectthe Rayleigh-like feature and, for example, only contribute to forming alow-angle scattering cone around the main light beam direction. Forexample, a low-angle diffuser layer comprising particles with dimensionslarger than the one of the nanoparticles and selected in size anddensity to contribute to forming a low-angle scattering cone around themain light beam direction may be provided as a separate layer or may beintegrated within the chromatic diffusing layer.

The chromatic effect is further based on nanoparticles having arefractive index that is different than the refractive index of theembedding matrix. To scatter, the nanoparticles have a real refractiveindex n_(p) sufficiently different from that of the matrix n_(h), (alsoreferred to as host material) in order to allow light scattering to takeplace. For example, the ratio m between the particle and host mediumrefractive indexes

$\left( {{{with}\mspace{14mu} m} \equiv \frac{n_{p}}{n_{h}}} \right)$may be in the range 0.5≤m≤2.5 such as in the range 0.7≤m≤2.1 or0.7≤m≤1.9.

The chromatic effect is further based on the number of nanoparticles perunit area seen by the impinging light propagating in the given directionas well as the volume-filling-fraction f. The volume filling fraction fis given by

$f = {\frac{4}{3}{\pi\left( \frac{d}{2} \right)}^{3}\rho}$with ρ [meter³] being the number of particles per unit volume. Byincreasing f, the distribution of nanoparticles in the diffusing layermay lose its randomness, and the particle positions may becomecorrelated. As a consequence, the light scattered by the particledistribution experiences a modulation which depends not only on thesingle-particle characteristics but also on the so called structurefactor. In general, the effect of high filling fractions is that ofseverely depleting the scattering efficiency. Moreover, especially forsmaller particle sizes, high filling fractions impact also thedependence of scattering efficiency on wavelength, and on angle as well.One may avoid those “close packing” effects, by working with fillingfractions f≤0.4, such as f≤0.1, or even f≤0.01 such as f=0.001.

The chromatic effect is further based on a number N of nanoparticles perunit area of the chromatic diffusive layer in dependence of an effectiveparticle diameter D=d n_(h). Thereby, d [meter] is the average particlesize defined as the average particle diameter in the case of sphericalparticles, and as the average diameter of volume-to-area equivalentspherical particles in the case of non-spherical particles, as definedin [T. C. GRENFELL, AND S. G. WARREN, “Representation of a non-sphericalice particle by a collection of independent spheres for scattering andabsorption of radiation”. Journal of Geophysical Research 104, D24,31,697-31,709. (1999)]. The effective particle diameter is given inmeters or, where specified, in nm.

In some embodiments:

${{N \geq N_{\min}} = {\frac{2.07 \times 10^{- 29}}{D^{6}}{{\frac{m^{2} + 2}{m^{2} - 1}}^{2}\left\lbrack {meters}^{- 2} \right\rbrack}}},$(D given in [meters]) and

${{N \leq N_{\max}} = {\frac{1.21 \times 10^{- 27}}{D^{6}}{{\frac{m^{2} + 2}{m^{2} - 1}}^{2}\left\lbrack {meters}^{- 2} \right\rbrack}}};$for example,

${{N \geq N_{\min}} = {{{\frac{4.24 \times 10^{- 29}}{D^{6}}{{\frac{m^{2} + 2}{m^{2} - 1}}^{2}\left\lbrack {meters}^{- 2} \right\rbrack}\mspace{14mu}{and}N} \leq N_{\max}} = {\frac{9.27 \times 10^{- 28}}{D^{6}}{{\frac{m^{2} + 2}{m^{2} - 1}}^{2}\left\lbrack {meters}^{- 2} \right\rbrack}}}},$more specifically

${N \geq N_{\min}} = {{{\frac{8.99 \times 10^{- 29}}{D^{6}}{{\frac{m^{2} + 2}{m^{2} - 1}}^{2}\left\lbrack {meters}^{- 2} \right\rbrack}\mspace{14mu}{and}N} \leq N_{\max}} = {\frac{6.48 \times 10^{- 28}}{D^{6}}{{{\frac{m^{2} + 2}{m^{2} - 1}}^{2}\left\lbrack {meters}^{- 2} \right\rbrack}.}}}$

For example, for embodiments aiming at simulating the presence of a pureclear sky,

${{N \geq N_{\min}} = {\frac{8.99 \times 10^{- 29}}{D^{6}}{{\frac{m^{2} + 2}{m^{2} - 1}}^{2}\left\lbrack {meters}^{- 2} \right\rbrack}}},$(D given in [meters]) and

${N \leq N_{\max}} = {\frac{3.69 \times 10^{- 28}}{D^{6}}{{\frac{m^{2} + 2}{m^{2} - 1}}^{2}\left\lbrack {meters}^{- 2} \right\rbrack}}$such as

${{N \geq N_{\min}} = {{{\frac{4.24 \times 10^{- 29}}{D^{6}}{{\frac{m^{2} + 2}{m^{2} - 1}}^{2}\left\lbrack {meters}^{- 2} \right\rbrack}\mspace{14mu}{and}N} \leq N_{\max}} = {\frac{2.79 \times 10^{- 28}}{D^{6}}{{\frac{m^{2} + 2}{m^{2} - 1}}^{2}\left\lbrack {meters}^{- 2} \right\rbrack}}}},$more specifically

${N \geq N_{\min}} = {{{\frac{8.99 \times 10^{- 29}}{D^{6}}{{\frac{m^{2} + 2}{m^{2} - 1}}^{2}\left\lbrack {meters}^{- 2} \right\rbrack}\mspace{14mu}{and}N} \leq N_{\max}} = {\frac{2.06 \times 10^{- 28}}{D^{6}}{{{\frac{m^{2} + 2}{m^{2} - 1}}^{2}\left\lbrack {meters}^{- 2} \right\rbrack}.}}}$

In other embodiments aiming at minimizing the contribution of a specularreflected scene,

${{N \geq N_{\min}} = {\frac{2.79 \times 10^{- 28}}{D^{6}}{{\frac{m^{2} + 2}{m^{2} - 1}}^{2}\left\lbrack {meters}^{- 2} \right\rbrack}}},$(D given in [meters]) and N≤

${N_{\max} = {\frac{1.21 \times 10^{- 27}}{D^{6}}{{\frac{m^{2} + 2}{m^{2} - 1}}^{2}\left\lbrack {meters}^{- 2} \right\rbrack}}},$such as

${N \geq N_{\min}} = {\frac{3.69 \times 10^{- 28}}{D^{6}}{{\frac{m^{2} + 2}{m^{2} - 1}}^{2}\left\lbrack {meters}^{- 2} \right\rbrack}\mspace{14mu}{and}}$${{N \leq N_{\max}} = {\frac{9.27 \times 10^{- 28}}{D^{6}}{{\frac{m^{2} + 2}{m^{2} - 1}}^{2}\left\lbrack {meters}^{- 2} \right\rbrack}}},$more specifically

${N \geq N_{\min}} = {\frac{4.85 \times 10^{- 28}}{D^{6}}{{\frac{m^{2} + 2}{m^{2} - 1}}^{2}\left\lbrack {meters}^{- 2} \right\rbrack}\mspace{14mu}{and}}$${N \leq N_{\max}} = {\frac{6.48 \times 10^{- 28}}{D^{6}}{{{\frac{m^{2} + 2}{m^{2} - 1}}^{2}\left\lbrack {meters}^{- 2} \right\rbrack}.}}$

In general, any of the following factors may be applied as upper orlower limit, including that value or excluding that value respectivelyin the term

${\frac{factor}{D^{6}}{\frac{m^{2} + 2}{m^{2} - 1}}^{2}}:$

factor factor (e.g. for Nmin) (e.g. for Nmax) 4.24e−29 9.27e−28(1.04e−27) 8.99e−29 6.48e−28 (7.27e−28) 2.79e−28 3.69e−28 3.69e−28(4.14e−28) 2.79e−28 4.85e−28 (5.44e−28) 2.06e−28 9.27e−28 (1.04e−27)1.21e−27 9.48e−28 (1.06e−27) 1.17e−27 (1.31e−27) 9.72e−28 (1.09e−27)1.07e−27 (1.20e−27)

With respect to those physical parameters and their general interplay,it is again referred to, for example, WO 2009/156348 A1.

The macroscopic optical properties of the chromatic diffusing layerdisclosed herein can be described in terms of the two followingquantities:

(i) The monochromatic normalized collinear transmittance T(λ), definedas the ratio between the transmittance of the chromatic diffusing layer,without the contribution of scattered light—e.g. regular transmittance,and the transmittance of a reference sample identical to the chromaticdiffusing layer except for the fact that the diffusing layer does notcontain the nanoparticles having a size in the range from 10 nm to 240nm, i.e. the nanoparticles which are responsible of preferentiallydiffusing the short wavelengths of the impinging radiation.

(ii) The ratio γ between the blue and the red optical densities definedas: γ≡Log[T(450 nm)]/Log[T(630 nm)] that measures the capacity of thechromatic diffusing layer to provide chromatic separation between longand short wavelength components of the impinging radiation.

In some embodiments, the chromatic diffusing layer may have:

T(450 nm) in the range from 0.05 to 0.95, for example from 0.1 to 0.9such as from 0.2 to 0.8. For example for embodiments aiming atsimulating the presence of a pure clear sky, T (450 nm) may be in therange from 0.4 to 0.95, for example from 0.5 to 0.9 such as from 0.6 to0.8.

In embodiments aiming at reducing (e.g. minimizing) the contribution ofa specular reflected scene, T(450 nm) may be in the range from 0.05 to0.5, for example from 0.1 to 0.4 such as 0.2 up to 0.3.

With respect to the ratio γ between the blue and the red opticaldensities in some embodiments, γ may be in the range 5≥γ≥1.5, or even5≥γ≥2, or even 5≥γ≥2.5 such as 5≥γ≥3.5.

For completeness, inorganic particles may be used such as those thatinclude but are not limited to ZnO, TiO₂, ZrO₂, SiO₂, and Al₂O₃ whichhave, for example, an index of refraction n_(p)=2.0, 2.6, 2.1, 1.5, and1.7, respectively, and any other oxides which are essentiallytransparent in the visible region. In the case of inorganic particles,an organic matrix or an inorganic matrix may be used to embed theparticles such as soda-lime-silica glass, borosilicate glass, fusedsilica, polymethylmethacrylate (PMMA), and polycarbonate (PC). Ingeneral, also organic particles may be used. For the types of materials,it is further referred to EP 2 304 478 A1.

The shape of the nanoparticles can essentially be any, while sphericalparticles are most common.

As mentioned above, the nanoparticles and/or the matrix and/or furtherembedded particles may not—or may only to some limited extent—absorbvisible light. Thereby, the luminance and/or the spectrum (i.e. thecolor) of the light exiting the chromatic diffusing layer may only bevery little or not at all affected by absorption. An essentiallywavelength-independent absorption in the visible spectrum may beacceptable.

As illustrated above, the scattering aspects are related to a relativerefractive index between nanoparticles (or generally nanoscalescattering elements) and a host material. As described, nanoparticlesmay refer to solid particles. However, also optically equivalent liquidor gaseous phase nanoscale elements such as generally liquid or gasphase inclusions (e.g. nanodroplets, nanovoids, nanoinclusion,nanobubbles etc.) having nanometric size and being embedded in the hostmaterials. Exemplary materials that comprise gas phase inclusion(nanovoids/nanopores) in a solid matrix include aerogels that arecommonly formed by a 3 dimensional metal oxides (such as silica,alumina, iron oxide) or an organic polymer (e.g. polyacrylates,polystyrenes, polyurethanes, and epoxies) solid framework hosting pores(air/gas inclusions) with dimension in the nanoscale. Exemplarymaterials that comprise liquid phase inclusions include liquid crystal(LC) phases with nanometric dimensions often referred to as liquid phaseincluding nanodroplets that are confined in a matrix that commonly mayhave a polymeric nature. In principle, there is a large variety of LCscommercially available, e.g. by Merck KGaA (Germany). Typical classes ofliquid crystal may include cyanobiphenyls and fluorinated compounds.Cyanobiphenyls can be mixed with cyanoterphenyls and with variousesters. A commercial example of nematic liquid crystals belonging tothis class is “E7” (Licrilite® BL001 from Merck KGaA). Furthermore,liquid crystals such as TOTN404 and ROTN-570 are available from othercompanies such as Hoffman-LaRoche, Switzerland.

With respect to LC, an anisotropy in refractive index may be present.This may allow to use liquid crystal droplets dispersed in a solidtransparent host material as scattering particles in a nanosize range(e.g. for Rayleigh-like scattering). Specifically, one can set acontributing relative index of refraction by changing a voltage appliedacross the liquid crystal droplets, e.g. using a sandwich structure ofan polymer dispersed liquid crystal (PDLC) layer provided in betweenelectrical contacts (such as ITO PET films or ITO glass sheets) in asandwich structure and applying a voltage across the PDLC layer using apower source. Specifically, creating an electric field aligns the liquidcrystal orientations within distinct nanodroplets to some extent. Forfurther details on LC embodiments, it is referred to the above mentionedinternational patent application entitled “TUNABILITY IN SUN-LIGHTIMITATING LIGHTING SYSTEMS.

For example, for the chromatic diffusing layer, a wavelength dependentensemble light scattering cross-section amount is given by a specificselection of properties of the chromatic diffusing layer (27), whichaffect its optical properties, including: a refractive index of thenanoscale scattering elements, in particular an anisotropy in therefractive index and/or a refractive index of constituting matter of thenanoscale scattering elements, a size and/or a shape of the nanoscalescattering elements, in particular an anisotropy in the geometric shape,a refractive index of the host material, in particular an anisotropy inthe refractive index and/or a refractive index of constituting matter ofthe host material, a volume fraction between the nanoscale scatteringelements and the host material, and/or a layer thickness of thechromatic diffusing layer.

In some embodiments, the chromatic diffusing layer may comprise apolymer dispersed liquid crystal layer with liquid crystals embedded ina host polymer, wherein the liquid crystals form nanodroplets, areseparated by the polymer, and have an anisotropy in the index ofrefraction. It may further comprise a pair of areal electrical contactsfor providing an electric field for interacting with the liquid crystalswithin the nanodroplets, wherein the areal electrical contacts extend onopposite faces of the polymer dispersed liquid crystal layer and atleast one of the areal electrical contacts is configured to betransparent in the visible wavelength range.

In some embodiments, a mean size of the nanoscale scattering elementsmay be in the range from about 10 nm to about 500 nm such as in therange from about 20 nm to about 400 nm such as in the range from about30 nm to about 300 nm. A volume fraction between the nanoscalescattering elements, in particular between liquid crystal droplets, andthe host material may be in the range from about 30% to about 70% suchas in the range from about 40% to about 60%. A layer thickness of thescattering layer may be in the range from about 10 μm to about 500 μmsuch as in the range from about 20 μm to about 350 μm, e.g. in the rangefrom about 30 μm to about 200 μm or even in the range from about 50 μmto about 100 μm, and optionally the layer thickness may be defined byspacer elements and/or may have a variation in thickness less than 10%across an area of 10 cm×10 cm of the chromatic diffusing layer.

Although, herein the Rayleigh-like scattering is primarily disclosed inconnection with panel structures, in view of the cited disclosures, itis apparent that also other configuration such as film, coating,sandwich structures can apply in a planar or curved, transmitting orreflecting manner.

Referring again to FIGS. 3A and 3B, on collimation unit 45, chromaticdiffusing layer 27 may be directly applied, for example as a continuouslayer. In some embodiments, chromatic diffusing layer 27 may be providedon a support board such as a, e.g. planar, plastic, such as polymeric,or glass board. In other embodiments, the chromatic diffusing layer maybe configured strong enough to act as a support for the luminaire and inparticular the collimation unit 45 or a part thereof. As discussedabove, chromatic diffusing layer 27 is constructed such that itpreferentially scatters short-wavelength components of incident lightfrom collimation unit 45 with respect to long-wavelength components ofincident light from collimation unit 45. The scattered light is referredherein as diffuse light 31 and it is associated with a blue(short-wavelength) color assuming a given selection of the scatteringconditions of the nanoparticles.

In an exemplary viewing direction 73 of an observer is indicated. Inviewing direction 73, an observer will see the portion of diffuse light31 that is emitted in his direction because diffuse light 31 isessentially homogenously emitted in all directions from chromaticdiffusing layer 27. Clearly, those portions being emitted towards lightsource 25, e.g. collimation unit 45, may be lost or partly quasirandomly reflected. In addition, the observer may see—depending on hisposition—several direct light beams 29 of the transmitted light of lightsource 25. Light beams 29 are perceived as a common main light beam (seeFIG. 3C and related description) and are, for example, “yellowish” withrespect to the spectrum of the light of the light source due to thescattering of the blue components. The seen transmitted light is basedon that portion of incident light that is directed to or to some extentredirected by chromatic diffusing layer or some forward scatter to faceviewing direction 73 of the observer.

As a consequence of the embedded nanoparticles, that portion of theincident light, which is transmitted without being deviated byscattering interaction with chromatic diffusing layer 27, has a visiblespectrum that differs from the spectrum of the incident light in anassociated center of mass-wavelength because the spectrum is shiftedtowards longer wavelengths (i.e. to the red giving a yellow tone). Theportion of the incident light, which is subject to the essentiallyRayleigh-like scattering by the nanoparticles, is emitted in a diffusemanner, thereby leading to substantially homogeneous luminance in allthe directions pointing away from its surface.

In connection with FIGS. 3C and 3D, the appearance of luminaire 21 ofFIGS. 3A and 3B is explained. Specifically, FIG. 3C illustrates aschematic 3D-view of a room 60 being illuminated with luminaire 21,while FIG. 3D illustrates the appearance of luminaire 21 within aperture35 when looked at from below.

Diffuse light 31 is primarily generated by the light emerging fromcollimation unit 45. Diffuse light 31 will always be seen when lookingat aperture 35.

In addition, FIG. 3C illustrates a main light beam 66 originating fromaperture 35 of luminaire 21 and propagating along main light beampropagation axis 67. Main light beam 66 is composed of a plurality ofsub-light beams, i.e. the direct light beams 29, two of which areillustrated for illustration purposes although those direct light beamswould not be resolvable with in main light beam 66 during operation.

As already illustrated in FIG. 3B, direct light beams 29 also propagatealong the direction of main light beam propagation axis 67, if oneassumes proper alignment of the components of light source 21.

Together, light beams 29 form an essentially constant luminancedistribution over aperture 35 for main light beam 66. The divergence ofmain light beam 66 is substantially similar to the divergence of lightbeams 29, assuming a more or less homogeneous directionality of lightbeams 29.

An observer 70 positioned within main light beam 66 will see the directlight of those direct light beams 29 for which the divergence and theposition with respect to aperture 35 results in light falling into theeyes of observer 70.

Referring to the sun imitation concept illustrated in particular in FIG.3D, observer 70 sees a bright disk 68 (perceived as the “sun” underrespective conditions such as high luminance peak, respective limiteddivergence, homogenous blue background) surrounded by a weakerhomogeneous bluish background light (perceived as the “sky”), i.e.diffuse light 31. Due to the limited divergence of light beams 29 (beamangle in the range of, for example, below 4°) and the fact that theangular content is basically the same across the whole output surface ofthe luminaire, the sun will appear to move over the sky (as indicated byarrow 68′ in FIG. 3D, i.e. the sun follows observer 70 as for the realsun) when observer 70 moves across room 60, specifically across mainlight beam 66 as indicated in FIG. 3C by arrow 70′.

In other words, aperture 35 delimits a surface over which at each pointthe angular emission is narrow with an almost constant luminance (powerper unit area per unit solid angle) over the surface. Neglecting for thefollowing considerations chromatic diffusing layer 27, putting a screenin close proximity to aperture 35 will indicate a near fielddistribution, while putting the screen far from luminaire 21 will show afar field of light source 21. In the near field, a spot of the same sizeas the aperture 35 is formed that is homogeneously illuminated. In thefar field, a spot with essentially the shape determined by the angularcontent of the light passing the aperture 35 would be formed on thescreen (e.g. a round shape if the light is emitted inside a cone tomimic the sun).

In the cases of FIG. 3C, main light beam 66 will fall on the flooressentially in under near field conditions such that the shape of line72 (indicating the transition to the shadow outside main light beam 66)is a rounded rectangle, i.e. a rectangle with slightly rounded corners.Similarly, the shape of light beams 29 is indicated to have developedaccording to a conical distribution of light directions towards acircular beam cross-section, however, for typical indoor propagationdistances, as said before the overall shape of main light beam 66, inwhich the contributions of each single light beam 29 is not resolvable,is essentially closer to the nearfield distribution. The embodimentsdisclosed herein may allow to have, inside each collimated lightemitting region, the same angular content at each point. For example,this may roughly (almost) be true for the case of a lens forming animage at infinity of a source in its focal plane, or a CPC.

The near field distribution is the illuminance (power per unit area)distribution impinging on chromatic diffusing layer 27. As the angularemission of diffuse light from chromatic diffusing layer 27 is the samefor each of its points (approximately Lambertian emission), theluminance of the “sky” is directly proportional to this illuminancedistribution. Therefore, in order to have a uniform “sky”, theilluminance distribution in correspondence of chromatic diffusing layer27 should be homogeneous on the spatial scale resolvable by theobserver's eye.

If the angular content of light beams 29 is quite large (e.g. a cone of±5° or ±10°), a localized non-uniformity quickly mixes up duringpropagation and such non-uniformities may have been “washed away”already a few centimeter or millimeter in front of aperture 35. However,as for sun imitation, the angular content is selected to be very narrow(e.g. a cone of ±2° or less such as ±1°), possible non-uniformities mayrequire larger propagation distances to “wash away”. Thus, in generalthe illuminance distribution should be as smooth as possible.

Referring to FIG. 3E, illustrate the combination of sunlight-based lightsource units and artificial light generating units in an illuminationsystem 241. Illumination system 241 comprises a luminaire 243 with asunlight receiving light guide unit (see FIG. 3H) that may be configuredand operated similar to the one described above in connection with FIGS.3A to 3C. Illumination system 241 comprises further a sunlight receivingunit 245 that collects the light of primarily the sun during the day andguides the collected light to luminaire 243. Specifically, sunlightreceiving unit 245 comprises a collector system 247 and a plurality ofoptical fibers 249. Collector system 247 may be based on a plurality ofreflective or refractive optical elements, such as mirrors or lenses251, mounted, for example, outside a building, e.g. at the roof of thebuilding to collect natural light.

As illustrated in more detail in FIG. 3F, lenses 251, for example, focusincident sunlight 253 onto an inlet face at a fiber input end 249A offiber 249 such that collected light 253 propagates by internalreflection within fiber 249 to an outlet face at a fiber output end 249Bof fiber 249. In the exemplary embodiment of FIG. 3F, lens 251 isassociated with a fiber 249. In alternative configurations, each lens251 may be associated with a group of fibers or a fiber bundle 249′ asillustrate in FIG. 3G.

In general, sunlight receiving unit 245 provides a plurality of fiberoutput channels, wherein each fiber output channel comprises at leastone fiber output end 249B.

As further shown in FIG. 3E, a control unit 253 may be provided to takecontrol measures when operating illumination system 241. For example,control unit 253 may receive information from a light sensor 255positioned next to the plurality of lenses 251 via control lines 257.Accordingly, light sensor 255 receives information about the outdoorlight conditions.

Additional elements such as a common chromatic filter element 259 (orone for each fiber/fiber bundle) or a common shutter element 261 (or onefor each fiber/fiber bundle) may be provided between each lens 251(collimator element) and each fiber input end 249A. Chromatic filterelement 259 may be selectable from a group of filters and/orcontrollable in its transmission to stabilize the chromaticity and/orthe total amount of sunlight provided to the sunlight receiving lightguide unit. Shutter element 261 may be controlled to stabilize the totalamount of sunlight provided to the sunlight receiving light guide unit.Those elements may be also connected to control unit 253 via controllines 257. In addition, control unit 253 is connected via anothercontrol line 257 to luminaire 243 and/or the sunlight receiving lightguide unit.

Furthermore, exemplary positions of indoor light sensors 255′ within aroom 263 may be used alternatively or in addition to outdoor lightsensor 255. For example, an indoor light sensor 255′ may be positionedwithin a direct light component or outside the direct light componentbut still within the diffuse light component. Alternatively oradditionally, an indoor light sensor 255′ may be positioned at theceiling next to luminaire 243. It will be understood that those indoorlight sensors 255′ may receive information on the sun-imitating light,and/or the sky-imitating light.

The light sensors may be configured to detect the chromaticity and/orthe intensity of the collected natural light, the direct light componentand/or the diffuse light component, as well as the general illuminationlevel and chromaticity within room 263.

FIG. 3H illustrates a luminaire 21′ that is configured to at leastpartly operate on natural light collected by collector system 247.Luminaire 21′ corresponds essentially to the one explained in connectionwith FIGS. 3A and 3B. However, a subgroup of light emitting units 41 isreplaced by fibers 249 delivering collected natural light to light guideunit 43. Exemplarily, in FIG. 3H, opposing light sources are composed ofa fiber-based light input coupling assemblies 250 and light emittingunits 41.

With respect to fiber-based light input coupling assemblies 250, thefiber's output light may also be coupled into light guide unit 41 usingoptical elements such as lenses or CPCs as discussed in connection withFIGS. 4A to 4M. In particular, a fiber-based light input couplingassembly 250 may provide a fiber light input interface adapted to bemounted next to a fiber output end 249B. In some embodiments,fiber-based light input coupling assembly 250 may comprise a mount formounting fiber 249 such that fiber output end 249B can provide directlycollected natural light into light guide 43.

In some embodiments, selected light guide strips only receive naturallight, while others receive only artificially generated light (or both).

Moreover, an artificial light source may also couple light into lightguide unit 43 together with natural light by combining optical paths ofthe natural light and the artificial light. For example, at the fiberinput end 249A, an optical light mixing element 265 shown in FIG. 3G mayreceive light from an LED 267 as well as collected light 269.Alternatively, fiber output end 249B and an LES of, e.g. an LED 41 (notexplicitly shown in FIG. 3H), may be positioned next to each other toallow coupling of either or both types of light directly into lightguide unit.

Control unit 253 may be configured to compensate changes in thecollected natural light with respect to the illumination perceived by anobserver 271. For example during the course of the day, control unit maycontrol the filter(s), shutter(s), the artificial light generatingsources to compensate for changes between morning, noon, and evening, orchanges caused by cloud formation before the sun.

Thereby, the illumination may be operated more efficiently during theday as less energy is needed to generate the required amount of lightcoupled in light guide unit 43.

In the following, various components and configurations of the lightsource are described in connection with the respective figures.

Referring to FIG. 4A to 4M, the light emitting units are the primarylight sources for the luminaire and may be coupled to a lateral side ofthe light guide unit. The coupling may be performed in various mannersthat depend inter alia on the type of the light emitting unit and on thetype of the light guide unit. In general, the light is coupled into thelight guide unit such that TIR occurs for light propagation within thelight guide unit.

For example, as shown in the schematic side views of FIGS. 4A to 4D forLEDs as primary light sources, LEDs may be edge-coupled to a light guideunit in several ways. An LED 81A, 81B may be encapsulated using arefractive optic (FIG. 4A) or a reflective optic (FIG. 4B) for couplingto light guide unit 43, respectively. An LED 81C, 81D may benon-encapsulated and use a refractive (FIG. 4C) or reflective optic(FIG. 4D) for coupling to light guide unit 43, respectively.Furthermore, a non-encapsulated LED 81E may be butt-coupled to lightguide unit 43 (FIG. 4E).

In some embodiments, the primary light source may comprise laser diodes,in which case a collimating element may not be required. For example, alaser diode 83A may be butt-coupled to light guide unit 43 (FIG. 4F).However, other optical elements such as an anamorphic prism pair (notshown) may be used to condition the radiated laser beam in somedesirable fashion (e.g., elliptical to circular beam shape).

In those embodiments, an LED or laser diode emission is provided thatis, for example, symmetric with respect to the plane of light guide unit43 when being coupled thereto. Exemplarily, a main propagation direction85A prior the coupling is indicated in FIG. 4A that is parallel.

As will be explained below, selecting a respective directionality of thelight being coupled into light guide unit 43 may further improve howmuch light can be coupled out. For that purpose, instead of providing anLED or laser diode emission that is symmetric (for example essentiallyparallel) with respect to the plane of light guide unit 43 when beingcoupled thereto, a main propagation direction prior the coupling may beprovided that is tilted with respect to the main propagation directionwithin light guide unit 43, e.g. the plane of light guide unit in apanel-type embodiment.

For example, an LED 81F may be non-encapsulated and use a refractiveoptic (FIG. 4G) for coupling to light guide unit 43 under an angle thatessentially is close to the TIR angle. Similarly, a laser diode 83B maybe butt-coupled to light guide unit 43, where a lateral side 87A may betilted with respect to the plane of light guide unit 43 (FIG. 4H) or alateral side 87B may connect main front face 55A and main back face 55Borthogonally with respect to the plane of light guide unit 43 (FIG. 4I).Further details on this change in the input light central direction ofthe primary sources are disclosed in connection with FIGS. 9A to 9C.

As mentioned above, the light emitting units may be emitting in a widespectrum (white light LEDs) or in narrow spectra. In the latter case,they need not be all of similar emission wavelength. For example,different sets of primary LEDs (for example three sets) might beinterlace coupled along light guide unit 43 in order to provide colortunability.

For example, FIGS. 4K and 4L show a top view on a panel shaped lightguide unit 43 receiving light from three types of LEDs 89. For example,LEDs 89 might be blue-, green-, red-, or even white-emitting to providewide color gamut tunability.

In similar embodiments, laser diodes 90 with different emissionwavelengths might be used as shown schematically on FIG. 4L.

Of course, higher number categories of primary sources may be used,including 4-primary colors (e.g. blue, green, amber, red) or 5-primarycolors (e.g. violet, cyan, green, amber, red), or more.

The primary light sources may be 100% direct-emitting, or might involvedown-conversion material. For example, amber-emitting LEDs based ondown-conversion are commercially available (LXZ1-PL02 by Lumileds);also, various white-emitting LEDs are available.

Example configurations of primary light sources and down-conversionmaterials are listed below in Table I. Each of the light sources is acandidate as a primary emitter coupling into the light guide unit asdescribed above. Each of the down-conversion materials can be used todown-convert the primary light prior coupling into the light guide unit(e.g. creating a white light primary source) or can be used as acandidate to be incorporated as a secondary light source receiving lightfrom the light guide unit and emitting down-converted light to thecollimating element. Examples described herein are the down-conversiondisks provided on the light guide unit or at the input side of a CPC orTIR lens.

There are many other combinations possible that are not listed in theTable I. In Table I, various visible-spectrum color regimes are referredto according to peak emission wavelengths, as follows: violet 400 nm to440 nm, blue 440 nm to 480 nm, cyan 480 nm to 510 nm, green 510 nm to550 nm, yellow 550 nm to 580 nm, amber 580 nm to 610 nm, and red 610 nmto 700 nm.

Moreover, for white light sources, chromatic effects of absorption inthe light guide unit as well as chromatic reflective effects mayinfluence the perception in particular of the color of the direct lightbeams. For example, the reflection at the lateral faces or at therespective LED/laser diode may have a chromatic absorption-like effectin the spectrum within the light guide unit. The emission spectrum ofthe primary sources and generally the spectrum of the light interactingwith the extracting features may be adapted to the material chromaticabsorption as well as any chromaticity of the reflection. To compensatefor those wavelength dependent effects within the wave-mixing guideconfiguration, one may tailor the input spectrum accordingly.Specifically, one may tailor the input spectrum in order to have anoutput spectrum with high CCT, high color rendering index, and R9values. For example, one may shift the spectrum of a single source (orof multiple sources, if multiple sources contribute to the light beingpresent at the extraction feature) towards the violet (with increasedblue and red components) to compensate for an absorption concerningchromatic components.

TABLE I Example configurations of primary light sources anddown-conversion materials. Violet Blue Cyan Green Yellow Blue-pumped1-Phosphor B1P n/a LED or LD n/a Phosphor 1 Blue-pumped 2-Phosphor B2Pn/a LED or LD n/a Phosphor 1 Blue, Green, and Red Emitters BGR n/a LEDor LD n/a LED or LD n/a Violet-pumped 1-Phosphor V1P LED or LD n/a n/aPhosphor 1 Violet-pumped 2-Phosphor V2P LED or LD n/a n/a Phosphor 1Violet-pumped 3-Phosphor V3P LED or LD Phosphor 1 n/a Phosphor 2Blue-pumped 1-Phosphor + Red B1PR n/a LED or LD n/a Phosphor 1Blue-pumped 1-Phosphor + Amber B1PA n/a LED or LD n/a Phosphor 1Blue-pumped 1-Phosphor + Amber + Red B1PAR n/a LED or LD n/a Phosphor 1Four Primary Emitters BGAR n/a LED or LD n/a LED or LD n/a Five PrimaryEmitters BCGAR n/a LED or LD LED or LD LED or LD n/a Six PrimaryEmitters VBCGAR LED or LD LED or LD LED or LD LED or LD n/a Amber RedBlue-pumped 1-Phosphor n/a n/a e.g., Blue LED + YAG Blue-pumped2-Phosphor n/a Phosphor 2 e.g., Blue LED + YAG + CASN Blue, Green, andRed Emitters n/a LED or LD e.g., Blue and Green InGaN LEDs and RedAlGAInP LED Violet-pumped 1-Phosphor n/a n/a e.g., Violet LED +BaSrSiO:Eu Violet-pumped 2-Phosphor n/a Phosphor 2 e.g., Violet LED +BaSrSiO:Eu + CASN Violet-pumped 3-Phosphor n/a Phosphor 3 e.g., VioletLED + BaMgAlO:Eu + BaSrSiO:Eu + CASN Blue-pumped 1-Phosphor + Red n/aLED or LD e.g., Blue LED + YAG + AlGAInP LED Blue-pumped 1-Phosphor +Amber LED or LD n/a e.g., Blue LED + YAG + Amber LED Blue-pumped1-Phosphor + Amber + Red LED or LD LED or LD e.g., Blue LED + YAG +Amber and Red LEDs Four Primary Emitters LED or LD LED or LD e.g., InGaNblue and green, AlGAInP amber and red Five Primary Emitters LED or LDLED or LD e.g., InGaN blue, cyan and green, AlGAInP amber and red SixPrimary Emitters LED or LD LED or LD e.g., InGaN violet, blue, cyan andgreen, AlGAInP amber and red

FIG. 4M illustrates schematically the coupling of light in a specifictype of light guide unit that comprises a sequence of light guide strips91. For example, light guide strips 91 extend parallel with respect toeach other. Each light guide strip comprises opposing lateral end faces(all lateral faces together at one side may be understood as lateralcoupling face 47. In FIG. 4M, the configuration having one laser diodeor one LED per light guide strip 91 is indicated (schematic laser diodes93A in dashed lines). However, multiple light guide strips 91(neighboring or interlaced) may be connected by some light guidingfiber-and-splitter structure 95 to a single laser diode 93B, or singleLED.

The light source regions may be defined by light that has been activelytaken out of the TIR travelling through the light guide unit at the mainback face (herein referred to as reflection mode) or at the main frontface (herein referred to as transmission mode). In general, light sourceregions may be disk-like shaped or may have many other shapes.

In general, operational modes are either based on narrow spectralprimary light using down-conversion to generate a broad white spectrumor are based on broad white light primary sources that alreadyessentially contain all colors needed to resemble the sky-sun-likeillumination. In general, the herein described concepts of the lightsource may however also be applicable independently from sunlightimitation if, for example, directed light in the manner discussed hereinis needed in some other spectral property or, for example, without theeffect of the Rayleigh-like type diffuser.

Referring to the reflection mode, FIG. 5A illustrates a first embodimentof a down-conversion based configuration 101 in the region of a singlelight source region of many light source regions provided on light guideunit 43. Specifically, a disk-like down-converting material 103 isdeposited on main back face 55B of light generating unit 43 (i.e.opposite the light emitting main front face 55A). Material 103 acceptse.g. short wavelength light from light guide unit 43, converts thatlight into a broad spectrum of wavelength, and then emits down-convertedlight back inter alia towards light guide unit 43. The thickness ofmaterial 103 can be increased to that of opacity. Moreover, the backsideof material 103 may be attached to a heatsink 105 (e.g. made of Al orCu) for thermal management. Down-converted light will pass through lightguide unit 43 in a region proximal to material 103 and be collected by acollimating optic such as a full refractive CPC lens 107.

In other embodiments (or in addition), material 103 may be a scatteringmaterial (or include some scattering material) interacting with, forexample, wide spectral light of respective primary sources.

Referring in detail to FIG. 5A, material 103 contains down-conversionmaterials (and/or material with specific scattering properties). Primarylight is coupled into light guide unit 43 from a primary source (notshown) and is guided in light guide unit 43 until it impinges material103. In FIG. 5A, exemplarily a primary light ray 109 is shown that isconverted to new emitted ray 111 that exits light guide unit 43 and iscollected and focused/collimated by CPC lens 107. In some otherpropagation cases, primary light may be converted to a ray 113 (in FIG.5A travelling to the left), which remains trapped in light guide unit43. This ray 113 has a chance to escape light guide plate 43 uponimpinging another material at another light source region in a furtherscattering event. In FIG. 5A, this other region is material 103, asshown by ray 115, in this case escaping and being collected andcollimated as well by CPC lens 107.

In the geometry of the embodiment shown in FIG. 5A, the thickness oflight guide unit 43 drives the diameter and length of CPC lens 107.Specifically, it is desirable to have the light guide thickness as thinas possible. The latter may make laser diodes attractive for this typeof geometry because the laser light may be coupled into very thin lightguides (even thinner than 10 μm).

As indicated above, material 103 may emit light at the side, i.e.outside of light guide unit (e.g. scattered or emitted light from withinthe interior of the material due to its finite thickness—for example,phosphor compound layers may be 10 μm or more, or even up to a fewhundred μm thick). Therefore, it may be desirable to include areflective containment structure (not shown) around material 103, e.g.at its back side and the lateral sides to increase the emission backtowards light generating unit 43.

With respect to the extent of the light source region on main front face55A of light guide unit 43 in the embodiment of FIG. 5A, the same isrelated to the lateral size of material 103, the thickness of lightguide unit 43, which define the area on main front face 55A throughwhich light emitted from material 103 can be collected by CPC lens 107.Accordingly, the input size of CPC lens 107 depends similarly on thelateral extension of material 103 and the thickness of light guide unit43.

FIG. 5B illustrates another embodiment of a reflection mode geometry.Specifically, a plurality of angled reflective structures 117 isincorporated into light guide unit 43. For example, reflectivestructures 117 may be angled at 450 with respect to the light guideplane defined by light guide unit 43. Those angled surface portions maybe coated with a specular reflective material such as a reflective metal(e.g. Ag or Al) or a dichroic stack of layers. This geometry may havethe advantage that the in-coupled primary light (which may spread overan angular range within the light guide unit of below 40° such as belowor about 30° or 20°, e.g. about 10°) is turned by reflective structure117 in an imaging manner which preserves etendue. This may allow thefinal collimation within collimation unit 45 to be completed with aconventional secondary lens structure 119 only (as shown in FIG. 5B9,e.g. without the need for a primary optics such as a CPC lens shown inFIG. 5C.

With respect to the extent of the light source region on main front face55A of light guide unit 43 in the embodiment of FIG. 5B, the same isrelated to the lateral size of angled reflective structure 117, thethickness of light guide unit 43, and to some extent by the inputdivergence of the primary light source which define the area on mainfront face 55A through which light emitted from angled reflectivestructure 117 passes.

Referring to FIGS. 5A and 5B and the etendue consideration disclosedherein, as the light source region are defined on the front surface ofthe light guide unit, those regions may actually be larger than the areaof the extracting element (e.g. the area of the angled reflectivestructure 117 on the back surface) due to the broadening of the “beam”when crossing the light guide unit. Therefore, the ratio between thecollected area light source region and the area of the main front facemay slightly increase in those embodiments.

In the embodiment shown in FIG. 5C, the primary light is narrow inspectrum (e.g. emitted from a monochromatic source such as amonochromatic LED or laser diode) and thus needs to be down-converted.Accordingly, the embodiment of FIG. 5C represents a mixture ofreflective extraction and extraction at the interface to the CPClens/down-converting material. One may reduce the required amount ofphosphor material in order to reduce the finite thickness issues notedearlier. For example, the phosphor material may be placed in a cavity orhalf-cavity geometry of the CPC lens.

As illustrated in FIG. 5C, a thin (perhaps as low as a single mono-)layer of phosphor compounds 121 is deposited between a CPC lens 123 andlight guide unit 43. An exit face 125 of CPC lens 123 may be coated witha dichroic 127 that reflects the primary light wavelengths (e.g. blueishlight), thereby returning the light to the phosphor compound andintroducing more interaction with the phosphor compound. For example,blue primary light that impinges on the layer of phosphor compounds 121,but is not absorbed (ray 129) is reflected by the blue-reflectivedichroic stack back towards phosphor compounds 121.

With respect to the extent of the light source region on main front face55A of light guide unit 43 in the embodiment of FIG. 5C, the same isrelated to the lateral size of angled reflective structure 117, thethickness of light guide unit 43, and to some extent by the inputdivergence of the primary light source which define the area on mainfront face 55A through which light emitted from angled reflectivestructure 117 passes. The layer of phosphor compounds 121 is intended toconvert the light redirected by angled reflective structure 117, andtherefore essentially corresponds to the size of the light sourceregion, although it substantially does not contribute to the lightextraction.

In the embodiment of FIG. 5C, CPC lens 123 is only configured to providefor a part of the collimation. A secondary lens array (not shown and inprincipal similar to secondary lens structure 119 in FIG. 5B) wouldcomplete the respective collimating element of collimation unit 45. Thesecondary lens would result in lateral extensions between individuallenses in the range of the pitch between two light source regions, inthis case two reflective structures 117. In this case, light sourceregion 57 could be identified as the layer of phosphor compounds. Iflight guide unit 53 is thin (with respect to the size of light sourceregion 57) or if the angular fan is narrow, the size of the layer ofphosphor compounds 121 is comparable with the size of reflectivestructure 117.

In a related embodiment similar to the one of FIG. 5C, the primary lightmay be white light such that no need of down-conversion exist.Accordingly, also the dichroic reflector at the output side of CPC lens123 is not necessary. Nevertheless, a diffuser acting mainly in theforward direction may be provided at the input side of CPC lens 123 toslightly fill the angular input of CPC lens 123.

Referring to the transmission mode, FIG. 6A illustrates a furtherembodiment in which a down-converting material 131 is positioned at mainfront face 55A of light guiding unit 43, e.g. a disk-shaped phosphorcompound material is deposited on the light-collecting side. In thiscase, down-converting material 131 accepts light from light guide unit43 and converts/emits (or in the embodiment of a scatterer, scatters)the light either back into light guide unit 43, or out of light guideunit 43, e.g. into refractive full CPC lens 133. The size ofdown-converting material 131 is chosen for optical coupling of lightinto CPC lens 133. Light emitted (or scattered) out the main back faceside of the light guide unit may be redirected back towards the disk bya reflector 135. Such light which would escape out the other side oflight guide unit 43, and not through down-converting material 131,should be masked by a light-masking layer 137 in order to avoid unwantedeffects of uncontrolled light emerging from main front face 55A, as thismight spoil the sun simulation.

Light emitted (or scattered) out of light guide unit 43 from theproximity to down-converting material 131 is down-converted. Thedown-converted light is then collected by CPC lens 133 acting as thecollimating optic as described above. Referring in detail to FIG. 6A,down-converting material 131 contains, for example, down-conversionphosphors and/or scattering properties and has a thickness chosen tomaximize light generation into the collimating optic. A primary lightray 139, coupled into light guide unit 43 via a primary source (notshown) is guided in light guide unit 43 until it impingesdown-converting material 131, as that acts as a change in index ofrefraction and thus change in TIR conditions. Primary light ray 139 maybe converted to an emitted ray 141 that exits light guide unit 43, andis collected and focused/collimated by CPC lens 133. Another primarylight ray may be converted to a new emitted ray 143 which becomestrapped in light guide unit 43. This ray 143 may have a chance to escapelight guide unit 43 later upon impinging another down-convertingmaterial in a scattering event, as shown by ray 145, in this caseescaping and being collected and (partially) collimated by CPC lens 133.

In addition, emitted and or scattered rays 146 may escape light guideunit 43 out the main back face 55B, and may be turned back downwardtowards the light-emitting side of light guide unit 43 by reflector 135.A mask layer 148 may be used to block light from emitting from lightguide unit 43 in a (non-source) area that would not couple into CPC lens133.

With respect to the extent of the light source region on main front face55A of light guide unit 43 in the embodiment of FIG. 6A, the same isrelated to the lateral size of down-converting material 131 and therespective region in which the change in TIR conditions is providedbecause in that area light can interact with down-converting material131 and leave light guide 43. The input size of CPC lens 107 dependssimilarly on the lateral extension of down-converting material 131 andthe assumed Lambertian distribution of the down-converted light that isintended to be collected.

FIG. 6B illustrates another transmission mode geometry, wherein thelight source regions for emission are provided by an optical coupling147 of CPC lenses 149 to light guide unit 43. This geometry isattractive because CPC lenses 149 may be mounted in a self-alignedmanner to its “source” for collecting light being extracted from lightguide unit 43 due to the induced local break of refractiveindex-mismatch at the optical interface and respective local break inTIR condition. The self-alignment is due to the light source regionsbeing formed by optically coupling the CPC and light guide unit 43.

With respect to the extent of the light source region on main front face55A of light guide unit 43 in the embodiment of FIG. 6B, the same isrelated to the lateral size of CPC lens 149 and the induced breakdown ofTIR conditions because in that area light leaves light guide 43. Theinput size of CPC lens 107 is thus essentially the size of the lightsource region.

Assuming that there is no presence of scattering centers, the lightdistribution that fills CPC lens 149 depends on the in-coupled primarylight ray distribution. Depending thereon, it may be improving the farfield light distribution and the sun appearance as a consequence byhomogenizing the light distribution in down-stream secondary optics inorder to achieve the desired near-field and far-field images of the“sun.” For example, one may include some mild (forward, low-angle)scattering centers at the optical element/light guide unit interface151.

In addition, as indicated by dotted lines, a reflective structure 117may additionally be provided to increase the amount of light coupled outat the light source region.

In contrast to the full CPC of FIGS. 6A and 6B, a “short” CPC 153coupled with a secondary lens 155, e.g. a Fresnel lens, is shown in FIG.6C. As further shown and applicable to all the herein disclosedembodiments, it may be desirable to include vertical light bafflestructures 157 within the lens system to reduce stray light effects—inaddition to lateral baffle structures 159 discussed above.

With respect to the extent of the light source region on main front face55A of light guide unit 43 in the embodiment of FIG. 6C, the same isrelated to the lateral size of CPC lens 149 as for the embodiment ofFIG. 6B.

Another transmission mode geometry is schematically shown in FIG. 6D,using a lens to extract light from the light guide unit. In particular,the contacting optical element is a lens structure 161 formed or moldeddirectly into light guide unit 43. This creates an emission surface thatcan be coupled to a secondary lens structure 163 for final collimation.

With respect to the extent of the light source region on main front face55A of light guide unit 43 in the embodiment of FIG. 6D, the same isrelated to the lateral size of lens 161 and the induced breakdown of TIRconditions similar to the embodiments of FIGS. 6B and 6D because in thatarea light leaves light guide 43. The size of lens 161 is thusessentially the size of the light source region.

In some embodiments related to the one of FIG. 6D, the size and shape ofthe interface between lens structure 161 and light guide unit 43 may bespecifically selected. For example, a cylinder smaller in diameter thanthe lens may be used to connect the lens to the guide (not shown). Thecylinder would act as some sort of mixing rod, or it could containscattering particle acting mainly in the forward direction to fill theangular gap of incident light which is the input of the lens.

Referring to the herein disclosed fan concept, the propagation insidethe light guide unit may be within a narrow angular aperture close tothe TIR angle. Then, the angular content impinging on the extractingoptics will be centered around that angle close to the TIR angle. Using,forward scattering particles at the input side of any primary optic willincrease the filling of both the far field and the near field lightdistributions. It is noted that for bi- (or more-) directional lightpropagation within light guide unit, the angular content impinging onthe extracting optics will be centered around that angle for both (ormore) directions thereby filling the input aperture of the primaryoptics (lens as well as CPC lens) in a broader manner, thus the farfield image may be more homogeneous.

As will be apparent to the skilled person, the illuminance distributionjust downstream the secondary lens structure 163, and in generaldownstream the collimated light emitting region, should be designed tobe as uniform as possible, thereby avoiding or at least reducingunwanted lack of uniformity in the appearance if the collimated lightemitting regions can be resolved by an observer.

FIGS. 6E and 6F illustrate further implementations of baffle structures165 and 167 as they may be applied to lens array structures to reduce oreven avoid cross-talk between neighboring collimating elements.

Referring to FIGS. 7A and 7B, a further aspect of this disclosurerelates to color changing in the resultant highly directional beam. Forexample, two or more sets of primary light sources of different colorsare coupled to a light guide unit and are operated by separateelectronic drivers or driver channels, which are powered by an externalpower sources (e.g., mains, or batteries).

The exemplary embodiment of a light source shown in FIG. 7A isconfigured for mixing two different colors in light guide unit 43. Thelight is generated by two types of light emitting units 169A, 169B. Eachtype of light emitting units 169A, 169B is operated by a specific driverunit 171A, 171B receiving power from a power supply 173. Within lightguide unit 43, the light can interact with, for example, the secondarysource “disks” to result in a formation of individual localized lightsource regions, which may be smaller in size, and well homogenized. Thecolor of the secondary sources may be tuned by changing the drivingconditions of driver units 171A, 171B, e.g. by amplitude or pulse-widthmodulation. In general, the concept is extendable to more types/sets ofprimary sources, including three, four, five, six, or more sets ofprimary light emitting units that are controllable via the controlsystem to perform, for example, color balancing or color setting.

The light source designs described in connection with FIG. 7A may beused for providing simulated direct sunlight. The light source may becombined with a Rayleigh-like type diffuser, as described above, tosimultaneously generate simulated direct sunlight and (diffuse)skylight. Additional conditioning optics, such as spatial filtering (forreducing or absorbing possible stray light), or turning optics (such asprismatic sheets or tilting/steering optical layers configured to tiltdirect light to produce a sun beam at angles different from theorthogonal to the waveguide plane) for directing the high intensity beamat an angle with respect to normal to the luminaire aperture, e.g. fortilting the main light beam propagation axis with respect to the normalto the light guide output window if the collimating unit output light iscentered around such direction, may be provided between the light guideunit and/or the diffuser. As described above, the thickness of theoverall luminaire may, for example, be further decreased when using acollimating optics for the highly directional beam in the form ofFresnel lenses, or a Fresnel lens array.

In FIG. 7B, a similar implementation of the above color changingconcepts is illustrated for light guide units comprising a plurality oflight guide strips 91. For example, each light guide strip 91 may beend-coupled with an LED 169A′, 169B′ at each end. LEDs 169A′, 169B′have, however, a different emission color. For example, LED 169A′ may bewhite-emitting while the LED 169B′ is amber-emitting.

In the exemplary embodiment of FIG. 7B, the position of the white/amberLEDs 169A′, 169B′ is alternated as light guide strips 91 are laid outacross aperture 35 of the luminaire. Separate driver units 171A, 171B topower the different colored LEDs 169A′, 169B′ may be used. Moreover,driver units 171A, 171B may be operated according to informationprovided by a clock/controller 175 which can be networked as describedfurther below.

As shown in FIGS. 7A and 7B, light emitting units may be provided on twoopposing sides of the light guide unit, in order to improve uniformityof the light intensity across the aperture of the luminaire.Furthermore, bezels (not shown) may be provided to aperture the lightemitting face to avoid visibility of any unwanted light intensitynon-uniformities proximal to the edge-coupled light sources.

For further illustrating the color mixing, FIG. 7C is a top view of asmall portion of the emitting luminaire aperture 35. Light source region177 for emitting scattered and/or converted light (as described above)accepts rays 179A, 179B, which are collected and redirected by a shortCPC lens 181. Rays 179A, 179B may be of different colors. For example,ray 179A may be a white light distribution, and ray 179B might be anamber distribution. Additional rays (not shown) may be added fromadditional light sources. Some rays may be down-converted by luminescentmaterials related to that light source region 177 within the disk, whileothers will be scattered or redirected to short CPC lens 181 without adown-conversion process. For the wide range of combinations of lightsources it is referred again exemplarily to Table I.

In the following an embodiment of a luminaire is described in moredetail based on the concepts described above.

The luminaire comprises a light guide plate (LGP) having an areasufficient to provide a 2′×4′ illuminating aperture. Such LGPs arewidely produced today for the display industry as backlights for liquidcrystal display (LCD) televisions and monitors. The LGP may be made frompolymethyl methacrylate (PMMA), also known as acrylic, and suppliersinclude companies such Wooyoung, Radiant, Coretronic, Pontex, Kenmos,JinMinShang, GLT, Enplas, and Zeon. While acrylic is very common, it isalso possible to make LGPs from more robust materials, even glass.However, unlike for LCD applications, for the luminaire, there is nohigh-density of extraction features incorporated into the LGP, insteadlocalized light source regions are homogenously distributed over theLGP. A typical thickness for an LGP of the size required here may be 3mm, but thinner (down to about 1 mm) and thicker (larger 3 mm)configurations may be feasible.

For the generating the light source regions, down-conversion materialsare mixed into a suitable binder such as silicone and deposited onto theLGP in a periodic spacing and with uniform size. For example, thedown-conversion materials may be dispensed or ink-jet printed to providedisks comprising phosphor grains of green/yellow- and red-emittingphosphor compounds on the LGP. The disk diameters may be targeted at 100μm, and their pitch in a light emitting direction of the light emittingunits may be set to 5.8 mm, resulting in over 20 000 disks deposited onthe LGP. Alternatively, the LGP may be temporarily masked according tothe pattern above, and the phosphor materials may be spray-coated ontothe LGP. After removing the mask, phosphor materials are left behind inthe proper form factor. Referring to the in FIG. 5A described reflectionmode configuration, the thickness of the disks may be selected for thatof opacity, typically larger 10 μm. Or, the disk is backed by areflective material.

Referring to the transmission mode described in FIG. 6A, the thicknessis chosen to optimize light out-coupling, for example in the range of 10μm to 200 μm. In the various embodiments, the phosphor grain sizes,doping levels, absorption characteristics, and loading density in thebinder, determine the relative ratio of collected primary and generatedsecondary (down-converted) light. The ratio may be suitable tuned toprovide the desired spectral characteristics of the luminaire byrespective selection of the parameters such as thickness, lateralextension, color mix of primary light sources, and type ofdown-conversion.

Based on the light source regions, a multi-source lighting system isformed. The efficiency of the multi-source lighting system targeting aparticular spectrum can be shown to be:

${\eta = \left\lbrack {\sum_{i = 1}^{n}\left( \frac{f_{i}}{\eta_{i}} \right)} \right\rbrack^{- 1}},{{\sum_{i = 1}^{n}f_{i}} = 1}$where f_(i) is the optical power fraction, and mli the power conversionefficiency, of the i^(th) source. For a down-converted source, theefficiency must take into account the quantum efficiencies of thephosphors, η_(ph), the “quantum deficit” (ratio of primary pump centroidphoton energy to that of the down-converted spectrum, typicallyapproximated as the ratio of the peak wavelengths), and the opticallosses associated with the conversion, sometime called “packageefficiency” (see Krames et al., IEEE J. Display Technol. 3, 160-175,2007).

For a single primary source pumping a single down-converter, theefficiency may be written as:

$\eta = {{\eta_{p}\left( {f_{p} + {f_{s}\frac{\lambda_{s}}{\lambda_{p}}}} \right)}^{- 1}\eta_{o}}$where η_(p) and f_(p) are the power efficiency and optical powerfraction of the primary pump source, respectively, f_(s) is the targetedoptical power fraction of the secondary, or down-converted, light,η_(ph) is the quantum efficiency of the down-converter, λ_(p) and λ_(s)are the peak wavelengths of the primary and secondary emission spectra,respectively, and η_(o) accounts for other optical losses via theconversion process. For a typical system targeting “cool white”(correlated color temperature of 4000 K to 7000K), the primary leakagefraction may be about 30%, and for typical phosphor quantum efficienciesof 90%, and assuming other optical losses of about 10%, a peak emissionwavelength of 450 nm for the primary source and 560 nm for thedown-converter, respectively, one can estimate η ˜0.7 η_(p). Inphotometric terms, and for typical lumen equivalents for LED-basedspectrum for cool white color temperatures of 340 lm/Wopt, theefficiency can be estimated at η˜240 η_(p) lm/W. That is, for a primarysource efficiency of 50%, an overall light source efficiency includingdown conversion losses may be about 120 lm/W. Additional optical lossesfor the LGP/lens array system and electronic driver losses (15%), wouldresult in an overall lower luminaire efficiency. If the LGP/lens arraylosses may be less than 15%, the overall luminaire efficiency could beabove 85 lm/W, thereby meeting, for example, the DLC standard for a2′×4′ commercial indoor luminaire (see DesignLights Consortium™ ProductQualification Criteria, Table 4: Primary Use Requirements).

To obtain, for example, at least 3000 lm in the light emitted by thelight emitting units into the LGP, and assuming the optical losseslisted above, a total primary light source electrical power of about 30W or more is required, or about 15 or more optical Watts. This mayeasily be achieved by using thirty (30) Watt-class high power LEDs.Another alternatively is to under-drive high power LEDs, to increaseefficiency and lifetime, and reduce thermal load. For example, alongeach thinner edge of the LGP, 30 high-power blue-emitting LEDs, such asLumileds' LXZ1-PR01, may be mounted along with coupling optics to injecttheir primary light emission substantially into the LGP and within theangle necessary for guiding within the LGP (total of 60 LEDs, eachoutputting about optical Watt). Along the other two edges of the LGP,and between each LED, reflective material may be applied to avoid lightleakage, in particular in the case of a light guide unit not beingconfigured as an ensemble of light guide strips.

For the collimating optics, a CPC lens array, or short-CPC plus Fresnellens array, or TIR lens array, or TIR-lens plus Fresnel lens arraydesigned to produce, for example, a 2° beam from the 100 μm diametersources, is formed or molded for mounting proximal to the LGP. Alignmentbetween the disk sources and Fresnel lenses is performed to opticallycouple the light source regions with the respective collimatingelements.

For an exemplary transmission mode geometry, a reflector may be providedproximal to the upper side (main back face) of the LGP (or applieddirectly in the case of a low refractive index laminated layer on thatside of the LGP). A mask layer, which may be reflective on the top side,but absorbing on the bottom, is attached proximal to the bottom side(main front face) of the LGP (or applied directly in the case of a lowrefractive index laminated layer on that side of the LGP). In the casewhere a low refractive index layer is to be provided on the same side asthe disks, the laminate may be removed where the disks are to beapplied.

The elements above, and in addition a chromatic diffusing layer and(optional) beam-steering optics, may be assembled into a fixture housingand the primary light sources are connected to an electronic driverpowered by mains or by a battery system, for example.

When the primary light sources are powered up, their light is convertedinto secondary sources at the down-converting disks. The generatedsecondary light then is collected by the lens array and may be steeredby the steering optics through the chromatic diffusing layer to providea sun-like beam (e.g. of about 2° or 4° divergence), and more than 3000lm, at a color temperature of 4000 K to 7000 K. Moreover, a diffuselight is generated that emulates the sky, and has a much higher colortemperature. For the system described in above, the total luminairethickness would be driven by the LGP thickness (e.g. 3 mm), thecollimating lenses (242 mm full CPC, or about 10 mm to 15 mm if shortCPC plus Fresnel), the Rayleigh-like type diffuser (e.g. fewmillimeters), and steering optics (few millimeters). In summary, thetotal luminaire thickness may be less than 50 mm if, for example, theshort CPC lens plus Fresnel lens approach is employed.

In the following an embodiment of a luminaire is described in moredetail based on the concepts described above and applying the concept ofcolor tuning.

It is known that high contents of blue light at night may disrupt thehuman circadian system, leading to health issues such as increased ratesof cancer and diabetes, and disturbed sleep patterns (see Stevens, etal., “Meeting report: The role of environmental lighting and circadiandisruption in cancer and other diseases” 2007. Department of Neurology,Faculty Papers, http://jdc.jefferson.edu/neurologyfp/22). Thus, in someembodiments of the herein disclosed luminaires, the ability is disclosedto tune the outputted spectra to reduce, for example, the amount of bluelight at night.

In addition to the primary blue light sources (for example used in theabove described embodiment), yellow or amber emitting LEDs are providedas further light emitting units. For example, amber emitting LEDs arecommercially available with the same form factor as the blue-emittingones described above (see LXZ1-PL02 from Lumileds). The amber LEDs maybe interlaced with the other primary light sources, and have their ownseparate driver electronics or channel.

Referring to clock/controller 175 shown in FIGS. 7A and 7B, a daytimeand/or seasonal clock input can be provided to control the drivers ofeach set of LEDs, so that the beam in the primary color may be changedin a preferred manner. The controller may be connected to the internetby any number of means, including wired network or wireless (e.g., WiFi,Zigby Radio, Bluetooth, Bluetooth LE, etc.) connection. The internetconnection can provide information necessary to properly tuning thelight output and color of the luminaire. For example, it might providetime of season and time of day for the location (e.g., GPS) of theluminaire. The location of the luminaire can be set upon installation inthe hardware of the controller, or by means of some other type ofcommissioning.

For example, during the day, the primary blue-emitting light sources areon, and the amber LEDs are off. In the evening, it is the other wayaround. The observed effect will be the primary beam achieving anextremely warm color temperature, and the reduced scattering of amberphotons by the chromatic diffusing layer will mean the “sky” will“darken”. Both of these effects are desirable from a circadian point ofview as well as from standard design principles for lighting at night.

The lumen output of the amber beam may to be as high as the “sun” beamoperating during the day. The amber LEDs may interact with the secondarydisk sources not through down-conversion, but via scattering offphosphor particles and/or scattering features provided within the diskregion. Thus, they are collected by the collimating optics in the sameway that primary and down-converted light is in the case of the “daymode”.

It is noted that a similar approach can be used to mimic the presence ofthe moon, e.g. by choosing a second set of primary light sources thatwill approximate the lunar emission spectrum upon interaction with thesecondary sources. The luminaire can be designed to have any arbitrarytransition pattern between day and night modes, by providing suitablescripts based on the clock information provided.

In the above embodiments are examples for configurations that usesimultaneously mixing disparate light sources and then collimating therespective output light. That is, the herein disclosed concepts areconfigured inter alia for combining multiple light sources of varyingetendue, colors, and intensities, homogenizing their output, andlocalizing the output as input into an optical system (collimatingelement) that then focuses/collimates the output into a single narrowbeam of light.

To state another way, the herein disclosed concepts transform variouslight sources of small emitting area (a few square millimeter or less)and large emission solid-angle into a homogenized light source of largelight emitting face (greater than 100 cm²) and small emission solidangle. The small emission solid angle is, for example, less than 8°(full width at half peak) such as less than 4°, or even less than 2°.The large light emitting face may be greater than 100 cm² such asgreater than 0.5 m² or even larger.

As described for the coupling of the primary light into the light guideunit in context with FIG. 4M, the inventors realized that one may reducethe surface area of the light guide unit with respect to that of thelight extracting features (resulting in the light source regions) inorder to reduce optical losses within the light guide unit or at itsedges.

As illustrated in FIG. 8A, for that purpose, the light guide unit maycomprise light guide strips 91. Such light guide strips 91 may bemounted to or formed on a large area substrate 191 (larger than thelight source regions as an example for a support structure) that has alower refractive index than that of the light guide material and rigidlysupports the thin light guide strips. It is noted that this may changethe TIR angular condition, and influence the guided angular distributionconfiguration. In alternative embodiments, in particular in cases of astructured main back side 55B, a mount configuration may be mechanicallyconnected to strips 91 in a few selected localized points, for examplereflective structures may be configured as junction points. Themechanical connection may be performed by a pin-mounting structure that,for example, comprises a series of reflective/metal flat-ended pinssupporting the light guide at selected regions. Thereby, unwantedextraction in correspondence to junctions/pins may be prevented or atleast reduced. Furthermore, pins may be mounted in opposing(antagonistic) pairs thereby clamping and holding the strip.

For example, considering light source regions 57 of 100 μm diameterlinearly displace by a pitch of 5.8 mm along light guide strips 91 andthe task to achieve a 2° beam, each light guide strip 91 may be 3×3 mm²in cross section (or smaller) and run the length (or longer) than thedesired length of the luminaire. With this geometry, the fraction oflossy light-guide edge area is reduced essentially, for example by abouta factor of two for the specific example described above. Also, theoverall mean optical path length of coupled light within the light guidematerial is reduced. Therefore, losses due to internal absorption of thelight guide material are decreased.

Referring to FIGS. 8B to 8D, exemplary configurations of primary lightsources and the collimating unit are schematically indicated. Inparticular, FIGS. 8B to 8D illustrate the ratios between variouselements of the luminaire such as size of light source regions(illustrated as dots), linear strips 91 (distance depending on pitchbetween light source regions and orientation of collimated lightemitting regions, non-emitting regions, etc.

In general, a light emitting unit may be edge coupled to either end oflight guide strips 91, see also the alternative shown in FIG. 4M. In theexemplary embodiments of FIGS. 8B and 8C, LEDs 193 are non-encapsulatedand use a refractive CPC lens 195 for coupling light into respectivelight guide strips 91. In the exemplary embodiment of FIG. 8D,non-encapsulated LEDs 194 are butt-coupled to light guide unit 43.

Light source regions 57 are provided periodically along each strip 91 ata pitch p. However, light source regions 57 may be shifted betweenneighboring strips 91, for example, by half a pitch, i.e. by half thedistance between two light source regions 57.

The light from each light source region 57 is collimated by acollimating element that receives light emerging from a respective lightsource region and emits collimated light from a respective collimatedlight emitting region.

Referring specifically to exemplary embodiment of FIG. 8B, collimatedlight emitting regions 197 are shaped circular and located concentricwith the respective light source region 57. Accordingly, areas 199 thatdo not emit light are formed in-between collimated light emittingregions 197 on the light emitting face of the light source. Herein,areas 199 are also referred to as non-emitting regions.

Referring specifically to the exemplary embodiments of FIGS. 8C and 8D,collimated light emitting regions 201 are shaped hexagonal and locatedconcentric with the respective light source region 57. The hexagonalshape allows the formation of a light emitting face that is essentiallyfree of areas that do not emit light. The hexagonal shape may be arespectively cut lens or a respectively formed CPC lens. It is notedthat small areas in-between collimated light emitting regions 201 mayremain that do not be emitting light due to structural implementationsof the collimating elements.

In an exemplary configuration, full fan LEDs are butt-coupled to smalllight guide strips 91 similar to the configurations in FIGS. 4E and 8D.Such configuration may be a simple implementation from a manufacturingpoint of view having an efficiency that can be reasonably high. Forexample, for a 0.25 mm×0.25 mm LED and a light guide strip with across-section of 0.3 mm×0.3 mm, an output of about 74.8% was simulatedwith about 24.8% being absorbed at the LED faces and about 0.4% beingabsorbed by the PMMA light guide (assuming 20% absorption at the LEDfaces). The extraction took place at light source regions having adiameter 100 μm (assuming 100% extraction when a light ray hits thelight source region) and the pitch between linearly aligned light sourceregions was 2.9 mm allowing a beam divergence down to 4°.

In similar exemplary configurations, full fan LEDs are coupled to smalllight guide strips 91 of about 60 cm length with a cross-section of 0.3mm×0.3 mm by means of an optical pyramid configuration. For a 0.25mm×0.25 mm LED, an output of about 75.6% was simulated for lightallowing a beam divergence of 2° under etendue considerations with about23.7% being absorbed at the LED faces and about 0.7% being absorbed bythe PMMA light guide (assuming 80% diffuse reflection at the LED faces).The extraction took place at light source regions having a diameter 100μm (assuming 100% extraction when a light ray hits the light sourceregion) and the pitch between linearly aligned light source regions was5.8 mm. For a 4° beam divergence and a respective pitch of 2.9 mm, anoutput of about 81.7% was simulated with, about 17.9% being absorbed atthe LED faces and about 0.4% being absorbed by the PMMA light guide.Along the light guides, the Illuminance distribution just outside thelight source region varies in an acceptable range such as down to 70% inthe center of the light guide for the 2° configuration.

For completeness, it is referred again also to FIG. 3A illustratingsquare shaped collimated light emitting regions 61. While in FIG. 3A noshift between the positions of the light source regions on neighboringstrips 91 is indicated, in particular the square shape allows forselecting various shifts without introducing additional non-emittingregions to the light emitting face.

As will be understood by the skilled person based on the hereindisclosed concepts, multiple colors may be provided, for example, byhaving different color emitting light emitting units at either end ofeach strip 91. Alternatively, the colors of light emitting units may bevaried strip 91 by strip 91 in embodiments having a pitch that is notresolvable by an observer by eye in a usual observer condition, e.g. fora very narrow pitch. In some embodiments, more primary light sources perstrip 91 may be added if the width W of strips 91 is allowed toincrease.

FIGS. 9A to 9C illustrate the concept of coupling a narrower angular faninto the light guide unit. In general, a narrower angular fan may becombined with a tilt of the input light central direction. It was foundthat in some configurations the use of a less divergent beam as inputmay increase extraction efficiency. The tilted angular fan may beconical (i.e. symmetric around its input light central direction) orasymmetric (e.g. adapted to the shape of the light guide strip). For theextraction, the tilt and fan in the plan orthogonal to the mainfront/back face is considered to be of primary interest and is shown inthe following figures schematically.

FIG. 9A illustrates the fan concept for a transmission modeconfiguration, that is, for example, similar to the one of FIG. 6B orFIG. 6C. A CPC lens 203 is optically coupled to light guide unit 43,thereby defining a light source region 57. Assuming coupling to thelight guide unit from opposite sides, at the light source regions, lightfrom both sides will see the change in TIR-conditions (as a local breakof the TIR condition because of a change in the geometry), thereby beingextracted from light guide unit 43.

In FIG. 9A, two counter-propagating light portions 205 are indicated.Due to the fan concept, the light within light guide unit 43 can beunderstood as having propagation directions within an angular range 207around an input light central direction 209. A tilt between input lightcentral direction 209 and a central axis 211 of light guide unit 43 isindicated by an angle θ, giving in general the angle of input lightcentral direction 209 with respect to the light guide unit plane (panelshape of light guide unit) or linearity (strip shape of light guideunit). In FIG. 9A, extracted light 213 is further indicated thatsimilarly populates a limited range of directions with respect to themain front face 55A of light guide unit 43, the direction of light 213is emphasized upwards as it would be for example the case after a firstcollimation optic (e.g. the embodiment of FIG. 6D).

In other words, the fan concept may allow providing a directionality tothe extracted light such as a spread of propagation directions of theinput light in ranges of ±10°, ±5°, ±4°, or ±2°. Moreover, one may varyinput light central direction 209 with respect to the light guide unitessentially up to the TIR angle to optimize light extraction, e.g. byincreasing the probability to interact with the extracting surface dueto increased TIR interaction on the main front face and/or the main backface of the light guide unit (than it would take place for lightpropagating along the central axis).

Light propagation simulations were used to evaluate the extractionefficiency for a 1 mm×1 mm×61 cm PMMA light guide strip. Specifically,the light guide strip was at first assumed to receive light from LEDsvia a coupling CPC lens (a coupling area of 0.8 mm×0.8 mm was consideredto result in a wide range of light propagation directions within thelight guide) distributed along the central axis, and to emit light atextractors extracting 100% of the incident light over an area with adiameter of 100 μm at a pitch of the extractors of 2.9 mm. Depending onthe PMMA absorption of the light guide strip and the LED-re-absorptionat the opposite side output coupling efficiencies larger than 50% weresimulated in particular for realistic absorption values for theLED-re-absorption at the opposing sides of the light guide strip.

When considering the same parameters but provide for tilted propagationof a fan-like shaped input light distribution around an input lightcentral direction of 42° (with respect to the strip direction) and aspread of ±5°, the light output may be increased significantly, to aboutfor example 80% and more. It is noted that geometrical losses at theextrema of the light guide unit due to the tilted direction of the lightinjection may need to be considered, as discussed below.

Thus, the “tilted” propagation of an input light fan may in particularbe advantageous when a refractive element is used for extraction asdescribed above.

FIG. 9B and FIG. 9C illustrate the fan concept for a reflection modeconfiguration, that is, for example, similar to the one of FIG. 5B orFIG. 5C. Similar to FIG. 9A, counter-propagating light portions 205 mayinteract with both sides of a, for example symmetric, reflective element215 that is used to redirect light of the TIR-conditions for lightextraction.

In FIG. 9B, the use of an angular fan centered around the direction ofcentral axis 211 is disclosed. Assuming reflection under 45°, extractedlight 213 may be extracted essentially orthogonally to main front face55A. An orthogonal extraction may simplify the design of the collimationoptics. In addition, a narrow fan may provide for a reasonable pitch ofthe re-collimating lens because an effective collection of the extractedlight may require smaller optics. The symmetric configuration withrespect to counter-propagating light portions 205 further may simplifydefining sun-like appearance of the light source regions/collimatedlight emitting regions as described in connection with FIGS. 10A to 10D.

In general, reflective extraction allows to enforce some directionalityon the extracted light. This is in particular can be used with tiltedfan configurations. Some aspects thereof are illustrated in FIG. 9C,

For “tilted” propagation inside the guide, the use of reflective elementfor extraction may provide for a tilted output beam, while maintaining asteeper angle of incidence. As shown in FIG. 9C, an asymmetricreflective element 217 comprises two faces 219A, 219B. Assuming thosefaces extend symmetric with respect to main back face 55B, depending ontheir angle and the tilt angle, extracted light 213 will comprisedivergent portions (as in FIG. 9A) or essentially parallel portions (asin FIG. 9B).

Providing differing inclination angles for faces 219A, 219B, adirectionality of the sunlight-imitating beam formed by the extractedlight 213 may be achieved. Having a tilted fan configuration may ensurethereby a steeper angle of incidence on face 219B than compared to anon-tilted fan configuration (indicated by face 219B′ in FIG. 9C). Thus,the combination of a tilted fan configuration with inclined sun beamimitation may be performed under improved reflective conditions andshaping the extracted light profile.

Similarly, providing different tilting angles and fan ranges for thecounter-propagating light portions 205 may be used to tune the system.

Light propagation simulations were used to evaluate for a 2 mm×2 mm×61cm PMMA light guide strip the extraction efficiency for tilted angularfans of varying angular range and propagation direction. Specifically,the light guide strip was assumed to receive light from angles close toTIR angle at angular ranges of, for example, ±5°, ±4°, ±3°. For anassumed reflectivity at the ends of 98% and a series of 1 mm×1 mm squareabsorbers with 100% absorption at a distance (pitch) of about 3 mm, andextraction features as stated before, a light output of up to and morethan 75% was calculated.

For completeness, it is noted that for a tilted angular fan situation,larger losses may occur at the opposite end of the light guide unit.

In several embodiments disclosed herein, down-converting elements suchas phosphor compounds or quantum-dot structures are used to generate awider spectrum. It is noted that the down-converting elements can beconsidered essentially as point sources for the respective collimatingelements. A down-converting element can be configured as a white lightemitter. However, not all emitted light may be used as some emissionwill partially go in the “wrong” direction. That lost emission maypartially be guided to another extracting feature or be absorbed in abaffle structure.

In particular when provided with lateral super-white reflectivematerial, phosphor compounds may act as mixing chambers having a lateraldimension and/or a thickness in the range from 5 μm to 300 μm such as 10μm to 100 μm. Examples include YAG-phosphor plus binder or a monolayerof a granular phosphor compound e.g. attached to the PMMA material of aCPC lens by adhesive.

With respect to reflective extraction, light from within the light guideunit may be reflected into a CPC/lens or onto a down-converting element.As an example, reflective prisms may be formed on the main back side ofthe light guide unit, for example reaching into the light guide unit.Essential for the perception, the shape of the reflective elementsshould be designed in retrospective from the collimating element. Forexample, the shape may be round to be sun-like in appearance. Reflectivestructures may further be mirror coated to increase reflectivity orprovide a focusing surface shape to direct more light, e.g., onto thedown-converting element.

As indicated above, in a reflection mode configuration not employing anydown-conversion, e.g. due to the use of broad white light primarysources, the shape of the reflecting surface may contribute to theappearance of the collimated light emitting face in the near field aswell as in the far field. The inventors realized that in particular fora configuration of counter-propagating light within the light guide unit(as discussed for example in context with FIGS. 9A to 9C), aconfiguration of the extracting feature should consider thiscontribution specifically when using a respective light source forsunlight imitation.

In connection with FIGS. 10A to 10D, a respective shape of a reflectiveelement is disclosed and simulations of the respective near field andfar field distributions are discussed.

Similar to the embodiments of FIGS. 5C and 5D, FIG. 10A illustratesexemplarily a light guide strip 91 having for light extraction providedon its main back face 55B a sequence of reflective prisms 221. Lightguide strip 91 may have dimensions of 1 mm×1 mm×61 cm and receive lightfrom LEDs 223 coupled to light guide strip 91 via CPC lenses 225 atrespective lateral coupling faces 47.

Reflective prisms 221 may have a lateral size of 100 μm and slope anglesof 45° to direct light components travelling parallel to light guidestrip's central axis essentially in an orthogonal extraction direction222A with respect to main front face 55A. The pitch between neighboringreflective prisms is, for example, 2.9 mm.

In the simulations, LEDs 223 are considered to be Lambertian emittersinto CPC lens 225 and to be 80% reflectors (performing diffusedreflection) when hit by (non-extracted and non-absorbed) light rayshaving passed through light guide stripe 91.

As illustrated in FIG. 10B, reflective prisms are reflectors that have atriangular cross section when looked at from the side and a round crosssection when looked at from main front face 55A, i.e. from therespective collimating element. Accordingly, each reflecting side 227A,227B of reflective prism 221 is shaped half-circle like such as ahalf-ellipse like face, thereby contributing to one half circle at theinput side of the respective collimating element.

Examples for collimating elements have been described above and includere-collimating CPC and/or Fresnel lens combinations placed with theiraxes, for example, aligned to an axis 229 of reflective prism 221.Although various exemplary embodiments were disclosed in the context ofCPC lens configurations, the skilled person will acknowledge thoseembodiments in which CPC lens configurations may be replaced by TIR lensconfigurations.

The shape of the angular distribution after the re-collimation lensdepends on the shape of the extracting surfaces, as seen by thecollimating element. As reflective prism 221 is “cut” so that inprojection it exhibits a round shape, an almost round far fielddistribution 231B can be obtained after the re-collimation, as shown inFIG. 10D.

To reduce or even avoid light being deflected by curved lateral surfaces233 of reflective prisms 221 to escape in the ambient and contribute tothe light emitted from the light source, absorbers should be placed nextto the faces of light guide strip 91 and surrounding the respectivecoupling CPCs.

An exemplary near field distribution 231A of one single extractionfeature is shown FIG. 10C based on gray scale intensity values. The sizeof the spot is of the order of 3 mm×3 mm. It fills almost completely thesquare “pixel”. A modulation of the intensity within the pixel is,however, still present. Inside the 3 mm×3 mm pixel, there seem not to beany zones without light.

Downstream the first CPC, a secondary re-collimation lens, or are-collimation lens alone obtains the desired narrow angle.

In FIG. 10D, far field 231B is shown after e.g. a Fresnel lens. Farfield 231B is essentially round, even if it exhibits a “ring”modulation, which may be caused by a slight defocus of the extractingfeature not being in the exact focal plane of the lens. Probably, if theluminance is high enough, the modulation may not be resolved because ofthe glare of the “sun” image.

According to simulations of the optical system, for the 61 cm×1 mm×1 mmlight guide stripe with 80% LED reflectance, with 0.7 mm LEDs sourcescoupled to the strip by CPCs, the output is simulated to be roughly 80%in the case of “square” prism reflectors (i.e. slightly less than for0.8 mm LED sources coupled via CPCs). With the“round-when-seen-from-above” prism reflectors the output is similarlyslightly lower than for the 0.8 mm sources condition.

Referring to the light source regions 57 and non-source regions 59 ofmain front face 55A, any unwanted flux from non-source region 59 may beabout or less than 10%, such as about or less than 5% or such as 1% orless of the flux originating from a light source region (57).

The flux behavior may be measured just outside of the main front face ofthe light guide unit, i.e. at a surface (or even a virtual surface inthe cases such as those using TIR frustration for light extraction) infront of the main front face and very close to it, e.g. at distances ofe.g. 10 μm or less or even in contact with the main front face.

It is noted that the visibility of structures does not depend entirelyon the flux magnitude. For example, also a quite weak light spotextremely concentrated at a certain angular range at large angle mayresult in a perceivable spot when looking at the luminaire and maygenerate a distortion of the sun imitations because a luminous whitespot appears outside the “sun”. Accordingly, the unwanted flux ofnon-source region 59 may not show highly localized spots or angularlylocalized spots. For example the variation of the unwanted flux may varynot more than resulting in local average flux densities of in average,e.g. 1% or less of the average flux of the light source regions.

Referring to configurations with down-converting elements, a properalignment may be required as in particular it is essential to aligndown-conversion elements with respective collimating elements. Forexample, the down-conversion element may need to be positioned in theinput side or within input side of a micro-CPC lens.

In general, the collimating unit may be a lens-array-structure, or amicro-lens-array structure and displaced large-lens-array structure, ora CPC-lens structure, or a short CPC-lens structure and displacedlarge-lens-array structure (herein also referred to as combinedCPC-lens) or a TIR lens structure. In particular, the combined CPC-lensstructure may reduce the thickness of the collimation unit. One mayassume that the inlet side of a micro-CPC lens may act as a point sourceof light that is collimated to e.g. an output angle (external, full) of30° with respect to a large secondary lens such as a Fresnel lens.

In some embodiments, a baffle structure may be positioned between(micro-) lenses or CPC lenses. The material for the collimating elementsmay be a PMMA structure having lateral air gaps between the neighboringPMMA structures. Lens, and generally optical element, structures and inparticular CPC lens structures may be printed, e.g. printedCPC-array-structures or printed micro-CPC-array structures.

An input side of a CPC lens may be sized in the range from 10 μm to 200μm in diameter such as 50 μm to 100 μm CPC entrance opening. An outputside of a CPC lens, and a large lens in general, 0.5 mm to 10 mm such as3 mm or 5 mm. Assuming an angular distribution at the input face of theCPC lens that does not completely fill the angular aperture of the CPClens, holes may occur in the output far field. For example, a hole in acentral zone may be generated due to missing normal incidence light onthe light guide main front face.

In general, the focal length of the second large lens may be in therange of the thickness of the collimation unit, e.g. in the range from 5mm to 100 mm such as 10 mm or 20 mm.

In some embodiments, the coupling of a CPC lens to the light guide unitmay be a TIR frustration coupling or a glue connection. Prior contactingthe CPC lens array and the light guide unit, the light guide unit may beprovided with a black layer having larger than needed holes at theintended contact position. In that case, the input sides of the CPClenses may define the sizes and positions and in general the lightsource regions.

In general, the collimation unit's collimating elements, and inparticular the large lenses, may be configured to provide for a tiltedlight beam originating from a collimated light emitting region. Forexample, one may configure the second large lens as off-axis lens withrespect to the extracting feature, or the first lens or (micro-) CPC,e.g. one may arrange the Fresnel lens off-axis. Such a misalignmentbetween the extracting feature or the CPC and the Fresnel lens mayresult in a tilt of the collimated light beam in desired few degreesrange with respect to propagation normal to the guide front face such asa tilt of about 15° with respect to the normal of the aperture of theluminaire.

It is noted that a homogenous emission corresponding to a homogenousluminance distribution over substantially the light emitting face is tobe understood over an area range that depends, for example, on theobserver distance, the type of structure and size of the collimatedlight emitting regions. In general, a homogenous emission may beachieved by arranging the collimated light emitting regions “as tight aspossible”, providing a light intensity within the light guide unit thatis essentially constant, e.g. by coupling light into light guide stripsfrom two sides etc., and coupling similar amounts of light into eachcollimating element. Moreover, the skilled person will understand thatthe light source regions may be provided substantially uniform e.g. in asubstantially regular pattern such as a regular grid structure over thenon-source region of the complete main front face. In some embodiment,the light source regions may be regularly provided in at least onedimension such as along a length along a light guide strip.

In some embodiments, the main front face may be split into severalsections, and the light source regions may be provided substantiallyuniform e.g. in a substantially regular pattern with respect to thesections of the main front face. For example, sections may be due tomounting requirements and the transition between sections may be coveredby a frame element. Similarly, multiple light sources may be combinedand interact with a common or respective chromatic diffusing layer. Foran architectural/mounting frame (e.g. white) separating those sections,the distance between the sections and thus their sizes dependent on thetype of implementation and observer distance (the longer the distancethe larger the minimum distance/size). For example, for indoorapplications (not to large rooms), section sizes having a lateral extendin at least one direction may be in the range from 7 cm and larger, forexample larger than about 20 cm. For large ambient installations, e.g.in large rooms or even at outdoor providing for example, 5 m to 10 mdistances to the luminaire), section sizes having a lateral extend in atleast one direction may be in the range from 10 cm and larger, forexample larger than about 30 cm.

It is further noted that a small LED emitting surface size may reducethe need of an angular fan emission, however, efficiently coupling intothe light guide may be more difficult for small LED emitting surfacesizes.

Moreover, it is noted that homogenizing layers such as forwardscattering layers or small-angle diffusers may be provided at thecollimating unit at various positions. For example, providing ahomogenizing layer at the input side of a CPC lens may result in a morehomogeneous near field and far field; providing a homogenizing layer atthe output side of a CPC lens, or at the second large lens layer or evenupstream or downstream the chromatic diffusing layer may furthersmoothen the emitted light. Thereby, structural features as generated,for example, by the non-emitting areas of the light emitting face may bereduced or even avoided in the perception of the luminaire's aperture.

With respect to the aspect of coupling light from the primary lightsources to the light guide unit, the embodiments disclosed hereinexemplarily relate to edge-lit light guide units, in particular primarylight is provided at a lateral coupling face connecting the main frontface and the main back face in a thickness direction. However, theskilled person will recognize that alternative coupling configurationsmay use the main front face and/or the main back face. For example, onemay use side-emitting LEDs, for example, for providing light into thelight guide unit at a boarder range of the main front face or main backface.

With respect to the aspect of the light guide unit, the embodimentsdisclosed herein exemplarily relate to guiding the light by totalinternal reflection, in particular due to respective selection ofrefractive index configurations. However, the skilled person willrecognize that alternative light guide configurations may be used that,for example, are based on reflective layers applied at least partiallyor section-wise on the light guide unit.

In the following various aspects are summarized. The skilled person willunderstand that feasible further developments and embodiments asdisclosed herein, and exemplarily identified in the dependent claims,are respectively applicable to those aspects identified in the summarysection as well as in the following.

(aspect) A light source for emitting collimated light in particular foran edge-lit large area luminaire, the light source comprising

a light guide unit having a main front face, a main back face, and atleast one lateral coupling face connecting the main front face and themain back face in a thickness direction, wherein

-   -   the light guide unit is configured for guiding light received at        the at least one lateral coupling face by total internal        reflection between the main front face and the main back face,    -   the main front face comprises a plurality of localized light        source regions distributed over a non-source region at equal        distances for having light pass there through, and    -   the ratio of the area of the plurality of light source regions        with respect to the area of the main front face (and/or: the        area of the non-source region) is less or equal to 20% such as        16% or 15% or less or less or equal to 10% such as less or equal        to a few percent such as less or equal to 5% such as less or        equal to 2% such as less or equal to 0.2%;

a plurality of light emitting units for emitting light into the lightguide unit through the at least one coupling face in the light emittingdirection;

a collimation unit extending along the main front face and comprising aplurality of collimating elements optically coupled to the plurality oflight source regions, wherein each collimating element is configured toreceive light emerging from a respective light source region and to emitcollimated light from a respective collimated light emitting region, thecollimated light emitting regions forming a light emitting face of thelight source.

(aspect) A light guide unit (43) comprising

a plurality of light guide strips (91), wherein the light guide stripsdefine together, for the light guide unit, a main front face (55A), amain back face (55B), and at least one lateral coupling face (47)connecting the main front face (55A) and the main back face (55B) in athickness direction (d_(T)), wherein each light guide strip (91) of theplurality of light guide strips (91)

-   -   is configured for guiding light received at the at least one        lateral coupling face (47) by total internal reflection, and    -   comprises a plurality of localized light source regions (57) at        the main front face (55A) for having light pass there through,        wherein the light source regions (57) are provided along the        light guide strip (91) within a non-source region (59).

(aspect) A light source—light guide unit comprising a light guide unitand

a plurality of light emitting units (41) (configured and arranged asdisclosed herein) for emitting light into the light guide unit (91)through respective portions of the at least one coupling face (47).

Further aspects are directed to a light source using optically separatedlight guide strips. The aspect may similarly be combinable with orwithout a liquid crystal based chromatic diffusing layer and/or a use ofcollected natural light. Specifically:

Aspect 1. A light source (25) for emitting collimated light (29) inparticular for an edge-lit large area luminaire (21), the light source(25) comprising:

a light guide unit (43) comprising a plurality of optically separatedlight guide strips (91), wherein the light guide strips define together,for the light guide unit, a main front face (55A), a main back face(55B), and at least one coupling face (47), wherein each light guidestrip (91) of the plurality of light guide strips (91)

is configured for guiding light received at the at least one lateralcoupling face (47), and comprises a plurality of localized light sourceregions (57) being areas in the main front face (55A) that are localizedin two-dimensions for having light pass there through, wherein the lightsource regions (57) are provided along the light guide strip (91) andare surrounded by a non-source region (59), anda plurality of light emitting units (41) for emitting light into thelight guide strips (91) through respective portions of the at least onecoupling face (47), wherein at least one light emitting unit (41) isconfigured as a light input coupling assembly (250) to receive collectednatural light from a fiber (249) and to provide the received naturallight to the light guide unit (43);a collimation unit (45) extending along the main front face (55A) andcomprising a plurality of collimating elements, wherein each collimatingelementcomprises an input side and an output side,is optically associated to one of the plurality of light source regions(57), andis configured to receive light emerging from the associated light sourceregion (57) at its input side and to emit collimated light (29) from arespective collimated light emitting region (61) formed at its outputside.

Aspect 2. The light source (25) of aspect 1, wherein the light guidestrips (91) receive light from light emitting units (41) at each end, inparticular at lateral coupling faces connecting the main front face(55A) with the main back face (55B) in a thickness direction (d_(T)),and/or

the plurality of collimating elements comprises, for each of theplurality of light guide strips (91), a plurality of compound parabolicconcentrators or TIR lenses having their input sides lined up at theside of the main front face (55A) at the positions of the respectivelight source regions (57) and optically coupled to the respective lightguide strip (91) for generating a TIR frustration for extracting lightfrom light guide unit (43).

Aspect 3. The light source (25) of aspect 1 or aspect 2, wherein atleast one of the plurality of light guide strips (91) further comprises,at the side of the main back face (55B), a plurality of reflectivestructures (117) respectively associated with the light source regions(177) of the respective light guide strip (91) and respectivelycomprising reflecting sides (227A, 227B) for reflecting lightpropagating along the light guide strip (91) from opposite ends onto arespective one of the reflecting sides (227A, 227B) through the lightsource region (57).

Aspect 4. The light source (25) of aspect 3, further comprising, at theside of the main front face (55A) of the at least one of the pluralityof light guide strips (91), a plurality of down-conversion elementsrespectively associated with one of the plurality of reflectivestructures (117) positioned to receive light reflected from therespective reflective structure (117) and having passed through thelight source region (57).

Aspect 5. The light source (25) of any one of aspect 1 to aspect 4,wherein the plurality of collimating elements comprises, for each of theplurality of light guide strips (91), a plurality of compound parabolicconcentrators or TIR lenses having their input sides lined up at theside of the main front face (55A) at the positions of the light sourceregions (57), and, for example, a plurality of secondary lensesrespectively associated with the compound parabolic concentrators or TIRlenses.

Aspect 6. The light source (25) of aspect 5, further comprising aplurality of down-conversion elements respectively associated with oneof the plurality of compound parabolic concentrators or TIR lenses andpositioned at the respective input sides, and wherein the plurality ofcompound parabolic concentrators or TIR lenses, for example, compriserespective dichroic reflective output sides reflective to the wavelengthof the light emitted from the light emitting units (41).

Aspect 7. The light source (25) of any one of aspect 1 to aspect 6,wherein the plurality of light emitting units (41) comprises a pluralityof light emitting diodes having a light emitting surface of about 0.25mm×0.25 mm, and the plurality of light guide strips (91) comprisesrectangular cross-sections in the range of about 0.3 mm×0.3 mm, andwherein in particular the light source regions (57) have each a circularshape with a diameter of about 100 μm and are, for example linearly,aligned along a respective light guide strip with a pitch in the rangeof about 3 mm to about 6 mm.

Aspect 8. The light source (25) of any one of aspect 1 to aspect 7,wherein the light guide unit further comprises a mounting structure formounting the light guide strips (91) such as a substrate or apin-mounting structure for supporting the light guide strips (91) at anextended area or at selected regions, respectively.

Aspect 9. The light source (25) of any one of aspect 1 to aspect 8,wherein the collimated light (29) of the plurality of collimatingelements forms a low-diverging light beam (66), having, for example, afull-width beam angle in the range of (below) 50 such as 4° or below,e.g. 2°.

Aspect 10. A large area luminaire (21) comprising

a light source (25) as in any one of the preceding aspect, and

a chromatic diffusing layer (27) comprising a plurality of nanoparticlesembedded in a matrix and configured to provide for a direct transmissionthat is larger in the red than in the blue and for a diffusetransmission that is larger in the blue than in the red, wherein thechromatic diffusing layer (27) is in particular positioned to byilluminated by the collimated light (29) or is positioned downstream ofthe light source regions (57) such as downstream of the input sideand/or the output side of the collimation unit (45).

Although the preferred embodiments of this invention have been describedherein, improvements and modifications may be incorporated withoutdeparting from the scope of the following claims.

The invention claimed is:
 1. A light source for emitting collimatedlight, the light source comprising: a light guide unit having a mainfront face, a main back face, and at least one lateral coupling faceconnecting the main front face and the main back face in a thicknessdirection (d_(T)), wherein the light guide unit is configured to guidelight received at the at least one lateral coupling face by totalinternal reflection between the main front face and the main back face,and the main front face comprises a plurality of localized light sourceregions that are areas with a limited extent in two-dimensions forhaving light pass there through, wherein the light source regions aresurrounded by a non-source region, a plurality of light emitting unitsconfigured to emit light into the light guide unit through the at leastone coupling face, wherein at least one light emitting unit of theplurality of light emitting units is configured as a light inputcoupling assembly to receive collected natural light from a fiber and toprovide the received natural light to the light guide unit; and acollimation unit extending along the main front face and comprising aplurality of collimating elements, wherein each collimating elementcomprises an input side and an output side, is optically associated toone of the plurality of light source regions, is configured to receivelight emerging from the associated light source region at its inputside, and to emit collimated light from a respective collimated lightemitting region formed at its output side.
 2. The light source of claim1, wherein the light source regions are provided in a substantiallyregular pattern over the non-source region in at least a section of themain front face, and/or wherein the regular pattern has a regular gridstructure that is based on one or more types of grid units, each type ofgrid unit having an essentially identical dimension and/or a shape of agrid unit comprising points at equal distances or being a triangulargrid unit or a square grid unit.
 3. The light source of claim 1, whereinat least one of at least one of the plurality of light emitting unitscomprises a light emitting device with a light emission over an angularrange that results in an angular range within the light guide unit ofbelow 40°, the angular range is characterized by an input light centraldirection and the input light central direction is inclined with respectto the normal to the main front face or main back face of light guideunit, and at least one of the plurality of light emitting unitscomprises a white light emission spectrum and/or a variation of emissionspectra.
 4. The light source of claim 1, wherein the ratio of the areaof the plurality of light source regions with respect to the area of themain front face and/or the area of the non-source region is less orequal to 16%.
 5. The light source of claim 1, wherein the light guideunit comprises two opposite lateral light coupling faces each beingassociated with a subgroup of light emitting units of the plurality oflight emitting units to thereby couple counter-propagating light intothe light guide unit.
 6. The light source of claim 1, wherein a lightemitting face is formed by the collimated light emitting regions, andthe light emitting face comprises non-emitting regions betweencollimated light emitting regions and the area ratio between thenon-emitting regions and the collimated light emitting regions isessentially constant over the light emitting face of the light source.7. The light source of claim 1, wherein at least one of the distancebetween neighboring light source regions defined as pitch distance in alight propagation direction is in the range from 0.5 mm to 50 mm, thelight source or the light guide unit is configured such that the flux oflight emitted from the light source regions is at least 10 times as muchas any leakage flux of light through the non-source regions, and thelight source regions are distributed within and substantially surroundedby the non-source region.
 8. The light source of claim 1, wherein theextension of the light guide unit in directions defined by the shape ofthe main front face is much larger than the thickness of the light guideunit in the thickness direction (d_(T)), and at least one of the lightguide unit is panel shaped having a thickness in the range from 1 mm to5 mm and a lateral extension in the range from 0.05 m to 3 m, and themain front face and the main back face are opposite and parallel to eachother.
 9. The light source of claim 1, wherein the light guide unitcomprises a plurality of optically separated light guide strips, whereinat least one of each light guide strip comprises at least one lateralcoupling face section optically coupled to at least one of the pluralityof light emitting units, and a subgroup of the plurality of light sourceregions arranged linearly along the light guide strip at equaldistances, the light guide strip has a rectangular cross-section with athickness in the range from 0.1 mm to 5 mm, a lateral width in the rangefrom 0.1 mm to 5 mm, and a length in the range from 0.1 m to 2 m, andthe plurality of light guide strips is mounted to a substrate at therespective sides of the light guide strips that are associated with themain back face of the light guide unit.
 10. The light source of claim 1,wherein the light source regions have a lateral extension in the rangeup to 2 mm, and/or the collimated light emitting regions have a lateralextension in the range from 0.5 mm to 50 mm.
 11. The light source ofclaim 1, further comprising at least one of a cross-talk reducing bafflestructure extending between and/or along collimating elements, therebyreducing the amount of light from a light source region entering acollimating element associated with a neighboring collimating element, afront leakage blocking structure extending between collimating elementsalong the main front face, and a back leakage blocking structureextending along the main back face and configured as a heat sink. 12.The light source of claim 1, further comprising a plurality ofreflective structures respectively associated with a light source regionand wherein at least one of a reflective structure of the plurality ofreflective structures is positioned and configured to reflect light topass through the light source region, a reflective structure of theplurality of reflective structures is positioned and configured toreflect light onto a respective down conversion element, and areflective structure of the plurality of reflective structures isconfigured as focusing reflector.
 13. The light source of claim 1,further comprising a plurality of reflective structures respectivelyassociated with a light source region and wherein at least one of areflective structure of the plurality of reflective structures ispositioned and configured to reflect light falling onto it from oppositedirections to pass through the light source region in a similarextraction direction, a reflective structure of the plurality ofreflective structures is configured as a reflective prism formed on themain back side, and a reflective structure of the plurality ofreflective structures has a triangular cross section when looked at fromthe side and a round cross section when looked at from the respectivecollimating element, and having a planar reflecting side that is shapedas a half-ellipse or as a half-circle.
 14. The light source of claim 1,wherein at least one of the plurality of collimating elements isconfigured to provide at least one main light beam propagation axis forthe propagation of the collimated light that is orthogonal or inclinedwith respect to the light emitting face, and wherein all collimatingelements are configured to provide for the same main direction.
 15. Asunlight-based illumination system for providing a direct light beam,and for generating an appearance resembling the sun within a sun-skyimitating illumination, the sunlight-based illumination systemcomprising: a sunlight receiving unit with a collector system, aplurality of optical fibers, wherein the collector system is configuredto collect natural outdoor light, and to couple the collected light intothe plurality of optical fibers, wherein each of the plurality ofoptical fibers comprises a fiber output end; and a light source of claim1 for emitting collimated light for an edge-lit large area luminaire,wherein the fiber output ends of the plurality of optical fibers arecoupled to light input coupling assemblies such that collected naturallight is coupled into the light guide unit and contributes to the directlight beam.
 16. An edge-lit large area luminaire comprising the lightsource of claim 1, and a chromatic diffusing layer comprising aplurality of nanoscale scattering elements embedded in a matrix andconfigured to provide for a direct transmission that is larger in thered than in the blue and for a diffuse transmission that is larger inthe blue than in the red, wherein the chromatic diffusing layer ispositioned to be illuminated by the collimated light or downstream ofthe light source regions.
 17. The edge-lit large area luminaire of claim16, wherein for the chromatic diffusing layer, a wavelength dependentensemble light scattering cross-section amount is given by a specificselection of properties of the chromatic diffusing layer, which affectits optical properties, including at least one of: a refractive index ofthe nanoscale scattering elements, and/or an anisotropy in therefractive index and/or a refractive index of constituting matter of thenanoscale scattering elements, a size and/or a shape of the nanoscalescattering elements, and/or an anisotropy in the geometric shape of thenanoscale scattering elements, a refractive index of the host material,and/or an anisotropy in the refractive index of the host material and/ora refractive index of constituting matter of the host material, a volumefraction between the nanoscale scattering elements and the hostmaterial, and a layer thickness of the chromatic diffusing layer. 18.The edge-lit large area luminaire of claim 16, wherein at least one of amean size of the nanoscale scattering elements is in the range fromabout 10 nm to about 500 nm, a volume fraction between the nanoscalescattering elements and the host material is in the range from about 30%to about 70%, a layer thickness of the scattering layer is in the rangefrom about 10 μm to about 500 μm, and the layer thickness is defined byspacer elements and/or has a variation in thickness less than 10% acrossan area of 10 cm×10 cm of the chromatic diffusing layer.
 19. Theedge-lit large area luminaire of claim 16, wherein the chromaticdiffusing layer comprises: a polymer dispersed liquid crystal layer withliquid crystals embedded in a host polymer, wherein the liquid crystalsform nanodroplets, are separated by the polymer, and have an anisotropyin the index of refraction; and a pair of areal electrical contactsconfigured to provide an electric field for interacting with the liquidcrystals within the nanodroplets, wherein the areal electrical contactsextend on opposite faces of the polymer dispersed liquid crystal layerand at least one of the areal electrical contacts is configured to betransparent in the visible wavelength range.
 20. The edge-lit large arealuminaire of claim 16, wherein the chromatic diffusing layer is providedas a panel that has a back side provided at the light emitting face ofthe light source, wherein the back side is configured to be illuminatedby incident light, and/or wherein the collimation unit comprises a firstcollimating element layer and a second collimating element layer and thechromatic diffusing layer is positioned between the first collimatingelement layer and the second collimating element layer, or wherein thecollimation unit comprises a first collimating element layer and asecond collimating element layer that is located downstream of the firstcollimating element layer and comprises the matrix and the plurality ofnanoparticles, or wherein the collimation unit comprises a coating withthe matrix and the plurality of nanoparticles applied to a lightemitting face formed by surfaces associated with the collimated lightemitting regions.
 21. The edge-lit large area luminaire of claim 16,wherein the light source and the chromatic diffusing layer areconfigured to provide for a light beam of non-diffused light with afirst correlated color temperature along a main light beam direction anddiffused light at a second correlated color temperature, wherein thelight beam is composed of a plurality of light beam components,respectively originating from respective collimated light emittingregions.
 22. The edge-lit large area luminaire of claim 16, furthercomprising a low-angle diffuser comprising particles with dimensionsselected in size larger than the nanoparticles, and density tocontribute to forming a low-angle scattering cone around the main lightbeam direction.
 23. A light source for emitting collimated light, thelight source comprising: a light guide unit having a main front face, amain back face, and at least one coupling face in a thickness direction(d_(T)), wherein the light guide unit is configured to guide lightreceived at the at least one lateral coupling face, and the main frontface comprises a plurality of localized light source regions that areareas with a limited extent in two-dimensions for having light passthere through, wherein the light source regions are surrounded by anon-source region, a plurality of light emitting units configured toemit light into the light guide unit through the at least one couplingface, wherein at least one light emitting unit in the plurality of lightemitting units is configured as a light input coupling assembly toreceive collected natural light from a fiber and to provide the receivednatural light to the light guide unit; and a collimation unit extendingalong the main front face and comprising a plurality of collimatingelements, wherein each collimating element comprises an input side andan output side, is optically associated to one of the plurality of lightsource regions, is configured to receive light emerging from theassociated light source region at its input side, and to emit collimatedlight from a respective collimated light emitting region formed at itsoutput side, and wherein the ratio of the area of the plurality of lightsource regions with respect to the area of the main front face and/orthe area of the non-source region is less or equal to 20%.